Nanofiber ribbons and sheets and fabrication and application thereof

ABSTRACT

The present invention is directed to nanofiber yarns, ribbons, and sheets; to methods of making said yarns, ribbons, and sheets; and to applications of said yarns, ribbons, and sheets. In some embodiments, the nanotube yarns, ribbons, and sheets comprise carbon nanotubes. Particularly, such carbon nanotube yarns of the present invention provide unique properties and property combinations such as extreme toughness, resistance to failure at knots, high electrical and thermal conductivities, high absorption of energy that occurs reversibly, up to 13% strain-to-failure compared with the few percent strain-to-failure of other fibers with similar toughness, very high resistance to creep, retention of strength even when heated in air at 450° C. for one hour, and very high radiation and UV resistance, even when irradiated in air. Furthermore these nanotube yarns can be spun as one micron diameter yarns and plied at will to make two-fold, four-fold, and higher fold yarns. Additional embodiments provide for the spinning of nanofiber sheets having arbitrarily large widths. In still additional embodiments, the present invention is directed to applications and devices that utilize and/or comprise the nanofiber yarns, ribbons, and sheets of the present invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application for patent claims priority to the following UnitedStates Provisional Patent Applications: 60/626,314, filed Nov. 9, 2004;60/666,351, filed Mar. 30, 2005; and 60/702,444, filed Jul. 26, 2005.

This work was supported by Defense Advanced Research Projects Agency/USArmy Research Office grant W911NF-04-1-0174, the Texas AdvancedTechnology Program grant 009741-0130-2003, and the Robert A. WelchFoundation.

FIELD OF THE INVENTION

Methods and apparatus are described for spinning high performancetwisted, false twisted and non-twisted yarns comprising nanofibers andfor drawing sheets and ribbons comprising nanofibers. Shaped articles,composites, and applications are described for these yarns, ribbons, andsheets.

DESCRIPTION OF THE BACKGROUND ART

Commercial synthesis methods produce nanofibers of either carbon singlewall nanotubes (SWNTs) or carbon multiwalled nanotubes (MWNTs) as asoot-like material. The strength and elastic modulus of individualcarbon nanotubes in this soot are well known to be exceptionally high,˜37 GPa and ˜0.64 TPa, respectively, for about 1.4 nm diameter SWNTs (R.H. Baughman, A. A. Zakhidov, and W. A. de Heer, Science 297, 787-792(2002)). Relevant for applications needing strong, but lightweightmaterials, the density-normalized modulus and strength of individualSWNTs are even more impressive, being factors of ˜19 and ˜54 higher,respectively, than for high-tensile-strength steel wire.

A critical problem hindering applications of these and other nanofibersis the need for methods for assembling these nanofibers into long yarns,sheets, and shaped articles that effectively utilize the properties ofthe nanofibers. Since such nanofibers can confer functionalities otherthan mechanical properties, methods are needed for enhancing themechanical properties of fibers made of the nanofibers withoutcompromising these other functionalities. Important examples of theseother functionalities, which combine with the mechanical functionalityto make the fibers multifunctional, are electrochromism, electrical andthermal conductivity, electromechanical actuation, and electrical energystorage.

Methods are known for growing both single wall and multiwalled nanotubesas forests of parallel aligned fibers on a solid substrate and forutilizing MWNT forests for a process to produce nanofiber assemblies (K.Jiang et al., Nature 419, 801 (2002)); and in U.S. Patent ApplicationPublication No. 20040053780 (Mar. 18, 2004). However, the resultingassemblies are extremely weak, so they cannot be used for applicationsthat require any significant level of tensile strength.

Although advances have been made in spinning polymer solutions orpolymer melts containing either SWNTs or MWNTs, the melt viscositybecomes too high for conventional melt or solution spinning when thenanotube content is much above 10%. Nevertheless, impressive mechanicalproperties have been obtained for polymer-solution-spun SWNTs, which inlarge part can be attributed to the mechanical properties of thenanotubes (see S. Kumar et al. Macromolecules 35, 9039 (2002) and T. V.Sreekumar et al., Advanced Materials 16, 58 (2004)). Another problemwith both polymer melt and polymer solution spinning is that thenanotubes are not present in sufficient quantities in the polymer toeffectively contribute to such properties as thermal and electricalconductivities. Additionally, the unique mechanical properties of theindividual nanotubes are diluted, since by far the major component ofthe fiber is polymer.

A. Lobovsky et al. (U.S. Pat. No. 6,682,677) have described asheath-core melt spinning process that attempts to avoid the usuallimitations caused by low concentrations of carbon nanotubes in meltspun yarns. This process involves melt compounding 30 weight percent ofvery large diameter carbon MWNTs (150-200 nm in diameter and 50-100microns in length) in a polypropylene matrix. This nanotube/polymermixture was successfully spun as the sheath of a sheath/core polymerthat contains polypropylene as the core. Despite the high viscosity ofthe nanotube/polymer mixture in the sheath and the brittleness of thesolidified composition, the presence of the polymer core permitted thissheath-core spinning and the subsequent partial alignment of nanotubesin the sheath. Pyrolysis of the polypropylene left a nanotube yarn thatis hollow (with outer diameter 0.015 inch and inner diameter 0.0084inch). To increase the strength of the hollow nanotube yarn, it wascoated with carbon using a chemical vapor deposition (CVD) process. Evenafter this CVD coating process, the hollow nanotube yarns had lowstrength and low modulus and were quite brittle (see Lobovsky et al. inU.S. Pat. No. 6,682,677).

A gel-based process enabled spinning continuous fibers ofSWNT/poly(vinyl alcohol) composites (B. Vigolo et al., Science 290, 1331(2000); R. H. Baughman, Science 290, 1310 (2000); B. Vigolo et al.,Applied Physics Letters 81, 1210 (2002); A. Lobovsky, J. Matrunich, M.Kozlov, R. C. Morris, and R. H. Baughman, U.S. Pat. No. 6,682,677; andA. B. Dalton et al., Nature 423, 703 (2003)). Present problems with thisprocess result from the fact that the nanotubes are simultaneouslyassembled in combination with poly(vinyl alcohol) (PVA) to form gelfibers, which are converted to solid nanotube/PVA fibers. This PVA theninterferes with electrical and thermal contact between carbon nanotubes.The PVA can be removed by thermal pyrolysis, but this severely degradesthe mechanical properties of the fibers.

Unfortunately, the polymer-containing fibers made by the above gelspinning processes are not useful for applications as electrodesimmersed in liquid electrolytes because they swell dramatically (by 100%or more) and lose most of their dry-state modulus and strength. Thisprocess means that these polymer-containing fibers are unusable forcritically important applications that use liquid electrolytes, such asin supercapacitors and in electromechanical actuators (R. H. Baughman,Science 290, 1310 (2000)).

In another process (V. A. Davis et al., U.S. Patent ApplicationPublication No. 20030170166), SWNTs were first dispersed in 100%sulfuric acid and then wet-spun into a diethyl ether coagulation bath.Though highly electrically conductive (W. Zhou et al., Journal ofApplied Physics 95, 649 (2004)), such prepared yarns have compromisedproperties, in part due to partial degradation of SWNTs caused byprolonged contact with sulfuric acid. This degradation, which can bepartially reversed by high temperature thermal annealing in vacuum,creates a serious obstacle for practical applications. Moreover, anysolution- or melt-based processing method that directly forms a polymerassembly is limited to short nanotube lengths (typically a few microns)by the viscosity increases associated with polymer dispersion andformation of globules having little nanotube orientation as a result ofnanotube coiling.

Y. Li et al. (Science 304, 276 (2004)) reported that MWNT yarns could beformed directly from unoriented carbon nanotube aerogels during nanotubesynthesis by CVD. While a twisted yarn was pictured, the ratio ofnanotube length (˜30 μm) to yarn diameter was about unity, which meansthat important property enhancements due to lateral forces generated bytwisting were not obtainable.

Twisting together micrometer-diameter fibers to make twisted yarnshaving enhanced mechanical properties is well known in the art, and hasbeen widely practiced for thousands of years. However, no successfulmeans has been conceived in the prior art for achieving the potentialbenefits of yarn twisting for nanofibers that are a thousand-fold ormore smaller in diameter than for the twisted yarns of the prior art.About a hundred thousand individual nanofibers would be in thecross-section of a 5 μm diameter yarn, as compared with the 40-100fibers in the cross-section of typical commercial wool (worsted) andcotton yarns. The challenge of assembling this enormous number ofnanofibers to make a twisted yarn having useful properties as a resultof a twist is enormous, and the teachings of the present invention willdescribe the structural features that must be achieved and how they areachieved.

Reflecting these problems with prior-art technologies of nanofiberyarns, important applications have not yet been commercially enabled,such as carbon nanotube artificial muscles (R. H. Baughman et al.,Science 284, 1340 (1999) and U.S. Pat. No. 6,555,945), carbon nanotubeyarn supercapacitors, structural composites involving carbon nanotubes,and electronic textiles involving strong, highly conducting nanofiberyarns,

No methods of the prior art have been developed for continuouslyproducing strong nanotube ribbons and sheets that are free of polymer orother binding agent, although said sheets would be quite valuable fordiverse applications. Carbon nanotube sheets of the prior art areusually made using variations on the ancient art of paper making, bytypically week-long filtration of nanotubes dispersed in water andpeeling the dried nanotubes as a layer from the filter (see A. G.Rinzler et al., Applied Physics A 67, 29 (1998) and M. Endo et al.Nature 433, 476 (2005)). Interesting variations of the filtration routeprovide ultra-thin nanotube sheets that are highly transparent andhighly conducting (see Z. Wu et al., Science 305, 1273 (2004) and L. Hu,D. S. Hecht, G. Grüner, Nano Letters 4, 2513 (2004)). Whilefiltration-produced sheets are normally isotropic within the sheetplane, sheets having partial nanotube alignment result from applyinghigh magnetic fields during filtration (J. E. Fischer et al., J. AppliedPhys. 93, 2157 (2003)) and mechanically rubbing nanotubes that arevertically trapped in filter pores (W. A. De Heer et al., Science 268,845 (1995)). In other advances, nanotube sheets that are either weak orhave unreported strengths have been fabricated from an un-orientednanotube aerogel (Y. Li, I. A. Kinloch, A. H. Windle, Science 304, 276(2004), by Langmuir-Blodgett deposition (Y. Kim et al., Jpn. J. Appl.Phys. 42, 7629 (2003)), by casting from oleum (T. V. Sreekumar et al.,Chem. Mater. 15, 175 (2003)) and by spin coating (H. Ago, K. Petritsch,M. S. P. Shaffer, A. H. Windle, R. H. Friend, Adv. Mat. 11, 1281(1999)).

For electrical device applications, nanofiber sheets are needed thatcombine transparency, electrical conductivity, flexibility, andstrength. Applications needs include, for instance light emitting diodes(LEDs), photovoltaic cells, flat panel liquid crystal displays, “smart”windows, electrochromic camouflage, and related applications.

Eikos, Inc. developed a transparent conductive coating based on carbonnanotubes (P. J. Glatkowski and A. J. David, WO2004/052559 A2 (2004)).They used solution-based technology involving carbon single wallnanotube inks. Transparent carbon nanotube (CNT) films involving apolymeric binder were made by N. Saran et al. (Journal American ChemicalSociety Comm. 126, 4462-4463 (2003)) using a solution deposition method.Also, transparent SWNT electrodes have been made by A. G. Rinzler and Z.Chen (U.S. Patent Application Publication No. US2004/0197546). A. G.Rinzler noticed high transmittance of a SWNT film in both the visiblerange and the near infrared (NIR) range (3-5 μm) (A. G. Rinzler and Z.Chen, Transparent electrodes from single wall carbon nanotubes,US2004/0197546)).

All of these processing methods are liquid based, and none providesstrong, transparent, nanofiber electrode materials or those that can beself-supporting when transparent. Also, none of these methods providesnanotube-based electrodes having useful anisotropic in-plane properties,like anisotropic electrical and thermal conductivity and the ability topolarize light.

There are reports about a successful application of a non-transparentcarbon nanotube film as a counter electrode in a Gräetzelphotoelectrochemical cell, which uses either liquid phase or solid phaseelectrolytes (see K.-H. Jung et al., Chemistry Letters 864-865 (2002);and S.-R. Jang et al., Langmuir 20, 9807-9810 (2004)). However, strong,transparent nanofiber electrodes have not been available for use in dyesolar cells (DSCs), although the need for them is apparent, particularlyfor flexible solid-state DSCs.

Additionally, none of the above-mentioned approaches have addressed theproblem of charge collection or injection from such transparent CNTcoatings into organic electronic devices: organic light emitting diodes(OLEDs), optical field effect transistors (OFETs), solar cells, etc.This problem requires either very low work function (w.f.) for electroninjection or high w.f. for hole injection.

Nanofibers, and in particular carbon nanofibers, are well known to beuseful as electron field emission sources for flat panel displays,lamps, gas discharge tubes providing surge protection, and x-ray andmicrowave generators (see W. A. de Heer, A. Chatelain, D. Ugarte,Science 270, 1179 (1995); A. G. Rinzler et al., Science 269, 1550(1995); N. S. Lee et al., Diamond and Related Materials 10, 265 (2001);Y. Saito and S. Uemura, Carbon 38, 169 (2000); R. Rosen et al., Appl.Phys. Lett. 76, 1668 (2000); and H. Sugie et al., Appl. Phys. Lett. 78,2578 (2001)). A potential applied between a carbon nanotube-containingelectrode and an anode produces high local fields as a result of thesmall radius of the nanofiber tip and the length of the nanofiber. Theselocal fields cause electrons to tunnel from the nanotube tip into thevacuum. Electric fields direct the field-emitted electrons toward theanode, where a selected phosphor produces light for a flat panel displayapplication and (for higher applied voltages) collision with a metaltarget produces x-rays for the x-ray tube application.

Methods are known for creating both single-wall and multiwall carbonnanotubes as forests of parallel aligned fibers on a solid substrate andfor utilizing such nanotube forests as cathodes (S. Fan, Science 283,512 (1999) and J. G. Wen et. al., Mater. Res. 16, 3246 (2001)). However,the resulting forest assemblies have various instabilities at largecurrent loads, one such instability being the flash evaporation ofcatalyst and carbon, followed by spark emission of light and by thetransfer of CNTs from cathode to anode, thereby destroying the cathode(R. Nanjundaswamy et. al., in Functional Carbon Nanotubes, edited by D.Carroll et al. (Mater. Res. Soc. Symp. Proc. 858E, Warrendale, Pa.,2005)). Although advances have been made in creating robust forests oforiented CNTs on glass substrates (e.g., by Motorola and Samsung), suchforests are still not the best solution for the nanofiber cold cathode.

One of the most challenging issues with oriented CNT arrays is theemission non-uniformity. Due to problems with screening effects andvariations in CNT structure and overall sample uniformity, only a verysmall fraction of the CNTs emit at any given time. Thus, unless specialtreatment is performed (e.g., chemical or plasma), emission from suchtypes of CNT forest cathodes is often dominated by edge emission and hotspots (Y. Cheng, O. Zhou, C. R. Physique 4, (2003)).

Stability is the second main technical issue which remains to be solved.Two primary reasons are usually responsible for the emissioninstability, namely the adsorption of residual gas molecules and Jouleheating of the CNTs (J.-M. Bonard, et al., Appl. Phys. Lett. 78, 2775(2001), N.Y. Huang et al., Phys. Rev. Lett. 93,075501 (2004)). Othermethods of making cold cathodes from CNTs include formation of acomposite with polymeric binder (O. Zhou et al., Acc. Chem. Res. 35,1045 (2002)) in which CNTs are not oriented. Nevertheless, impressiveemissive properties have been obtained for polymer binder/SWNT coldcathodes. The field screening effect seems not to play a crucial role inrandomly oriented CNTs simply due to their statistical distribution.Also, in these types of emitters, field-induced alignment is possiblethat might significantly enhance field emission properties. However, thesame problems that exist for oriented forests of CNTs also exist inthese types of emitters.

The problem with polymer binder/CNT cathodes is that the nanotubes arenot present in sufficient quantities in the polymer to effectivelycontribute to field electron emission and also to such properties asthermal and electrical conductivities (so that the binder is destroyedby heat and current). Additionally, the unique electrical properties ofthe individual nanotubes are diluted, since the major component of thecathode is by far the polymer binder. Thus, the upper level of stablefield emission current is significantly reduced.

A critical problem hindering applications of these carbon nanotubes(CNT) cold cathodes is the need for methods of assembling thesenanotubes into the framework of a macroscopic mounting system that issufficiently strong and suitably shaped such that the properties of theCNTs for field emission can be effectively utilized.

SUMMARY OF THE INVENTION

The present invention is directed to nanofiber yarns, methods of makingsaid yarns, and to applications of said yarns. Additional embodimentsprovide for the drawing of nanofiber ribbons, as well as sheets havingarbitrarily large widths. Importantly, this yarn spinning and sheet andribbon drawing technology can be extended to produce various yarns,sheets, and ribbons of diverse nanofiber materials for use in a varietyof applications and devices.

In some embodiments, processes of the present inventions for spinningyarns comprising nanofibers comprise the steps of: (a) arrangingnanofibers in an array selected from the group consisting of (i) analigned array, and (ii) an array that is converging towards alignment,so as to provide a primary assembly about whose alignment axis twist canoccur; (b) twisting about the alignment axis of said primary assembly toproduce a twisted yarn; and (c) collecting said twisted yarn via atechnique selected from the group consisting of (i) winding the twistedyarn on a spindle, (ii) depositing the twisted yarn on a substrate, and(iii) incorporating said twisted yarn into another structure; wherein(i) a significant component of the nanofibers have a maximum thicknessorthogonal to the nanofiber axis of less than approximately 500 nm, (ii)the nanofibers have a minimum length-to-thickness ratio in the thinnestlateral thickness direction of at least approximately 100, (iii) theminimum ratio of nanofiber length to yarn circumference is greater thanapproximately 5, and (iv) the net introduced twist in one direction peryarn length, compensated by twist in an opposite direction, for atwisted yarn of diameter D is at least approximately 0.06/D turns. Insome embodiments, the step of arranging involves a drawing process.

Prior to or after yarn collection, twist in one direction in a singlesyarn can be compensated by twist in an opposite direction at anotherstage in processing for a variety of useful purposes, such as for (a)plying yarns by folding them onto themselves and (b) forming compositeor welded structures in which the nanotubes are untwisted or minimallytwisted. The benefits provided by the initially introduced twist can beyarn densification and/or increases in yarn strength that enableapplication of increased forces on the yarn during initial processing.

In some embodiments, the present invention is directed to a process ofproducing a yarn comprising nanofibers, the process comprising the stepsof: (a) providing a pre-primary assembly, wherein the pre-primaryassembly comprises a substantially parallel array of nanofibers; (b)drawing from the pre-primary assembly to provide a primary assembly ofthe nanofibers having an alignment axis about which twisting can occur,wherein the primary assembly is selected from the group consisting of(i) an aligned array and (ii) an array that is converging towardalignment about the alignment axis; and (c) twisting about the alignmentaxis of said primary assembly to produce a twisted yarn.

In some embodiments, the present invention is directed to an apparatusfor producing a yarn comprising nanofibers, the apparatus operable toperform a process comprising the steps of: (a) providing a pre-primaryassembly, wherein the pre-primary assembly comprises a substantiallyparallel array of nanofibers; (b) drawing from the pre-primary assemblyto provide a primary assembly of the nanofibers having an alignment axisabout which twisting can occur, wherein the primary assembly is selectedfrom the group consisting of (i) an aligned array and (ii) an array thatis converging toward alignment about the alignment axis; and (c)twisting about the alignment axis of said primary assembly to produce atwisted yarn.

In some embodiments, the present invention is directed to an apparatusfor producing a yarn comprising nanofibers, said apparatus comprising:(a) a pre-primary assembly, wherein the pre-primary assembly comprises asubstantially parallel array of nanofibers; (b) a drawing mechanismattached to the pre-primary assembly, wherein the drawing mechanism isoperable to draw from the pre-primary assembly to provide a primaryassembly of the nanofibers having an alignment axis about which twistingcan occur, wherein the primary assembly is selected from the groupconsisting of (i) an aligned array and (ii) an array that is convergingtoward alignment about the alignment axis; and (c) a twisting mechanism,wherein the twisting mechanism is operable to twist about the alignmentaxis of said primary assembly to produce a twisted yarn.

In some embodiments, the present invention is directed to a process ofproducing a nanofiber ribbon or sheet comprising the following steps:(a) arranging nanofibers to provide a substantially parallel nanofiberarray having a degree of inter-fiber connectivity within the nanofiberarray; and (b) drawing said nanofibers from the nanofiber array as aribbon or sheet without substantially twisting the ribbon or sheet,wherein the ribbon or sheet is at least about one millimeter in width.

In some embodiments, the present invention is directed to an apparatusfor producing a nanofiber ribbon or sheet, the apparatus operable toperform a process comprising the steps of: (a) arranging nanofibers toprovide a substantially parallel nanofiber array having a degree ofinter-fiber connectivity within the nanofiber array; and (b) drawingsaid nanofibers from the nanofiber array as a ribbon or sheet withoutsubstantially twisting the ribbon or sheet, wherein the ribbon or sheetis at least about one millimeter in width.

In some embodiments, the present invention is directed to an apparatusfor producing a nanofiber ribbon or sheet, the apparatus comprising: (a)a substantially parallel nanofiber array having a degree of inter-fiberconnectivity within the nanofiber array; and (b) a drawing mechanism,wherein said drawing mechanism is operable for drawing nanofibers fromthe nanofiber array as a ribbon or sheet without substantially twistingthe ribbon or sheet, wherein the ribbon or sheet is at least about onemillimeter in width.

In some embodiments, the present invention is directed to a nanofibersingles yarn comprising about at least about a ten thousand nanofibersin a square micron of a cross-section of nanofiber singles yarn,wherein: (a) the nanofiber singles yarn is at least about one meter inlength; (b) the nanofiber singles yarn has a diameter less than aboutten microns; and (c) the nanofiber singles yarn is in the form selectedfrom the groups consisting of unplied, plied and combinations thereof.

In some embodiments, the present invention is directed to a processcomprising the steps of: (a) selecting a porous yarn comprisingnanofibers; (b) knotting the yarn to form a knotted yarn; and (c)obtaining a region-selective material manipulation of the yarn byexposing the knotted yarn to a substance selected from the groupconsisting of a gas; vapor; plasma; liquid; solution; fluid dispersion;super critical liquid; melt; conditions resulting in electrochemicaldeposition, electrochemical materials removal, electrochemicalpolymerization, and combinations thereof.

In some embodiments, the present invention is directed to a process formaking a deformable nanofiber sheet or ribbon comprising the steps of:(a) selecting a substrate from the group consisting of an elasticallydeformable substrate, an electrically deformable substrate, andcombinations thereof; (b) elongating the substrate to form a deformedsubstrate, wherein said elongation is selected from the group consistingof elastically elongating, electrically elongating, and combinationsthereof; (c) adhesively applying a nanofiber sheet or ribbon to thedeformed substrate; and (d) enabling at least partial return of saidelongation after said adhesive application step.

In some embodiments, the present invention is directed to a process forembedding an elastomerically deformable nanofiber sheet between twoelastomeric polymer sheets comprising: (a) selecting a first elastomericpolymer sheet; (b) elastically elongating the first elastomeric polymersheet to form a deformed substrate; (c) adhesively applying a nanofibersheet to the deformed substrate; (d) enabling at least partial return ofsaid elastic elongation after said adhesive application step; (e)applying a resin precursor for a second elastomeric polymer sheet to thenanofiber sheet when the first elastomer polymer sheet is in relaxed orpartially relaxed state; and (f) curing the resin precursor to form thesecond elastomeric polymer sheet while the first elastomer polymer sheetis in a relaxed or partially relaxed state.

In some embodiments, the present invention is directed to a process forspinning yarns comprising nanofibers, the process comprising the stepsof: (a) drawing a primary assembly comprising aligned nanofibers from aforest of nanofibers, wherein the angle between direction of drawing andalignment direction of the nanofibers in the forest is between aboutninety degrees and about five degrees; and (b) twisting the primaryassembly of nanofibers about an axis that is generally aligned with thenanofibers of the primary assembly to produce a nanofiber twisted yarn.

In some embodiments, the present invention is directed to process forspinning yarns comprising nanofibers, the process comprising the stepsof: (a) drawing from an array of as synthesized nanofibers to form aprimary assembly comprising a plurality of generally aligned nanofibers,wherein the nanofibers from the array of nanofibers are successivelylinked during the drawing step, have maintained previous linkages duringthe drawing step, or combinations thereof; and (b) twisting the primaryassembly of aligned nanofibers about an axis generally aligned with thenanofibers to produce a nanofiber twisted yarn, wherein length of thesignificantly prevalent nanofibers is at least five times thecircumference of the twisted nanofiber yarn.

In some embodiments, the present invention is directed to a process ofmaking twisted yarn comprising nanofibers, the process comprising thesteps of: (a) spinning a nanofiber yarn comprising at least 20% byweight nanofibers using a liquid-based method, and (b) twisting aboutthe yarn direction to provide a twisted yarn.

In some embodiments, the present invention is directed to a process forproducing a nanofiber ribbon or sheet from a nanofiber forest thatcomprises the following steps: (a) producing a nanofiber forestcomprising nanofibers, wherein the nanofiber forest is suitable fordrawing ribbons or sheets from the nanofiber forest, wherein the ribbonor sheet would be at least about one millimeter in width and wherein thenanofiber forest has a sidewall; (b) connecting an attachment to thesidewall or near the sidewall of the nanofiber forest, and (c) drawingthe nanofiber ribbon or sheet from the nanofiber forest by drawing uponthe attachment.

In some embodiments, the present invention is directed to a processcomprises the following steps: (a) producing a carbon nanotube forestcomprising nanotubes, wherein the carbon nanotube forest is suitable fordrawing ribbons or sheets from the carbon nanotube forest, wherein theribbon or sheet would be at least about one millimeter in width andwherein the carbon nanotube forest has a sidewall; (b) connecting anattachment to the sidewall or near the sidewall of the carbon nanotubeforest; (c) drawing the ribbon or sheet from the carbon nanotube forestby drawing upon the attachment, wherein the ribbon or sheet is a highlyoriented aerogel ribbon or sheet; and (d) infiltrating the sheet orribbon with a liquid and subsequently evaporating the liquid from thesheet or ribbon, wherein the infiltration and evaporation at leastpartially densifies the sheet or ribbon and forms a densified sheet orribbon.

In some embodiments, the present invention is directed to a process forstrengthening a yarn, ribbon, or sheet comprising nanofibers, whereinsaid process comprises the steps of: (a) infiltrating a liquid into theyarn, ribbon or sheet, and (b) evaporating the liquid from the yarn,ribbon, or sheet to strengthen the yarn, ribbon or sheet.

In some embodiments, the present invention is directed to process ofstrengthening a yarn comprising nanofibers, said process comprising thesteps of: (a) twisting the yarn in a first direction; and (b) twistingthe yarn in a second direction, wherein the second direction is oppositethe first direction and net twist of the twisting in the first andsecond direction is about zero.

In some embodiments, the present invention is directed to an apparatusfor producing a twisted nanofiber yarn, wherein the apparatus comprises:(a) a supply of nanofibers; (b) a transport tube for transporting thenanofibers from the supply to a collector; (c) the collector thatcollects nanofibers from the supply, wherein the collector is rotatable;(d) a winder that withdraws twisted nanofiber yarn from the collectorwhile the collector is rotated, whereby as the twisted nanofiber yarn iswithdrawn from the collector the nanofibers within the collector aretwisted to form twisted nanofiber yarn.

In some embodiments, the present invention is directed to a process ofproducing a twisted nanofiber yarn, wherein the process comprises: (a)continuously supplying nanofibers to a collector; (b) rotating thecollector to form an assembly of largely parallel nanofibers; (c)forming a nanofiber yarn from the assembly; and (d) withdrawing thenanofiber yarn from the assembly, wherein the yarn is twisted due to therotation of the collector to form a twisted nanofiber yarn.

In some embodiments, the present invention is directed to an apparatusfor producing a twisted nanofiber yarn, the apparatus operable toperform a process comprising the steps of: (a) continuously supplyingnanofibers to a collector; (b) rotating the collector to form anassembly of largely parallel nanofibers; (c) forming a nanofiber yarnfrom the assembly; (d) withdrawing the nanofiber yarn from the assembly,wherein the yarn is twisted due to the rotation of the collection toform a twisted nanofiber yarn.

In some embodiments, the present invention is directed to a devicecomprising an array of aligned conductive channels, wherein (a) saidconductive channels are operable for directional transport of speciesselected from the group consisting of electrons, ions, phonons, andcombinations thereof; and (b) said conductive channels are provided forby nanofibers in a form selected from the group consisting of ribbons,sheets, and combinations thereof.

In some embodiments, the present invention is directed to a methodcomprising the steps of: (a) providing oriented nanofibers in a formselected from the group consisting of ribbons, sheets, and combinationsthereof; and (b) using said oriented nanofibers as an array ofconductive channels for the directional transport of species selectedfrom the group consisting of electrons, ions, phonons, and combinationsthereof.

In some embodiments, the present invention is directed to a devicecomprising: (a) a cathode, wherein said cathode comprising nanofibers ina form selected from the group consisting of yarns, ribbons, sheets andcombinations thereof; and (b) an anode, wherein a region of low gaspressure separates said anode from said cathode.

In some embodiments, the present invention is directed to a devicecomprising: (a) a cold cathode, said cold cathode comprising nanofibersin a form selected from the group consisting of yarns, ribbons, sheets,and combinations thereof, wherein said form is made by a processcomprising the steps of: (i) arranging nanofibers in aligned arrayshaving sufficient inter-fiber connectivity within the array so as toprovide a primary assembly; and (ii) drawing said nanofibers as anelectrode material from the primary assembly; and (b) an anode, whereina region of low gas pressure separates said anode from said cathode.

In some embodiments, the present invention is directed to a process ofpatterning nanofiber sheets along their length, the process comprising apatterning technique selected from the group consisting ofphoto-polymerization, photolithography, electron-beam induced reactionof polymer; pressure-induced material transfer; liquid, gas phase, andplasma treatments to deposit, remove, and transform materials; andcombinations thereof.

In some embodiments, the present invention is directed to anoptoelectronic device comprising: (a) a first electrode, said firstelectrode comprising nanofibers in a form selected from the groupconsisting of yarns, ribbons, sheets and combinations thereof; (b) anactive layer operably associated with the first electrode; and (c) asecond electrode operably associated with the active layer and the firstelectrode.

In some embodiments, the present invention is directed to anoptoelectronic device comprising: (a) a first electrode, said firstelectrode comprising nanofibers in a form selected from the groupconsisting of ribbons, sheets, and combinations thereof, wherein saidform is made by a process comprising the steps of: (i) arranging thenanofibers in aligned arrays having sufficient inter-fiber connectivitywithin the array so as to provide a primary assembly; and (ii) drawingsaid nanofibers as an electrode material from the primary assembly; (b)an active layer operably associated with the first electrode; and (c) asecond electrode operably associated with the active layer and the firstelectrode.

In some embodiments, the present invention is directed to a method formaking an optoelectronic device, said method comprising the steps of:(a) providing components comprising: (i) a free-standing nanofibermaterial operable for use as a first electrode, wherein said material isin a form selected from the group consisting of yarns, ribbons, sheets,and combinations thereof, and wherein the material has athree-dimensional network of pores comprising a surface area in therange between about 100 m²/g and about 300 m²/g; (ii) an active materiallayer operably associated with the first electrode; and (iii) a secondelectrode operably associated with the active material and the firstelectrode; and (b) assembling the components to operatively form theoptoelectronic device.

In some embodiments, the nanotube yarns comprise carbon nanotubes. Suchcarbon nanotube yarns of the present invention provide unique propertiesand property combinations such as extreme toughness, resistance tofailure at knots, high electrical and thermal conductivities, highabsorption of energy that occurs reversibly, up to 13% strain-to-failurecompared with the few percent strain-to-failure of other fibers withsimilar toughness, very high resistance to creep, retention of strengtheven when heated in air at ˜450° C. for one hour, and very highradiation and UV resistance, even when irradiated in air. Furthermore,these nanotube yarns can be spun as one micron diameter yarns and pliedat will to increase the linear density (i.e., the weight per yarnlength) by forming two-folded, four-folded, and multi-folded yarns.

In some embodiments, the nanofibers are nanoscrolls. In someembodiments, the nanofibers are chemically and/or physically modifiedbefore or after a twist spinning or a ribbon or sheet draw process. Insome embodiments, the nanofiber yarns are used to form composites.

The nanofiber yarns of the present invention can be used in a variety ofdiverse applications. In some embodiments, this spinning technology canbe extended to produce various nanofibers and nanoribbons of diversematerials that can extend the range of applications. Applications forthe nanofiber yarns of the present invention include textiles;electronic devices; conducting wires and cables; electrochemical devicessuch as fiber-based supercapacitors, batteries, fuel cells, artificialmuscles, and electrochromic articles; field emission and incandescentlight emission devices; protective clothing; tissue scaffoldapplications; and mechanical and chemical sensors.

Advantages of the present invention will become more apparent from thedetailed description given hereinafter. However, it should be understoodthat the detailed description and specific examples, while indicatingpreferred embodiments of the invention, are given by way of illustrationonly, since various changes and modifications within the spirit andscope of the invention will become apparent to those skilled in the artfrom this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is an optical micrograph showing nanotube fibers being drawn froma nanotube forest, while twisted at high rate using the pictured motor.

FIG. 2 shows scanning electron microscope (SEM) micrographs, at twodifferent magnifications (A and B), of the consolidation of a drawnribbon into a twisted yarn during draw-twist spinning from a forest ofMWNTs, wherein an ˜600 μm wide forest strip formed the pictured 3.2 μmdiameter twisted nanofiber yarn.

FIG. 3 shows SEM pictures of (A) singles, (B) twofold, and (C) fourfoldcarbon MWNT yarns, as well as (D) knitted and (E) knotted carbon MWNTsingles yarns.

FIG. 4 provides SEM pictures showing that twisted carbon MWNT singlesyarn (bottom) and twofold yarn (top) retain twist up to the point wherefracture has occurred due to tensile failure.

FIG. 5 shows engineering stress-strain curves up to fracture for (a)carbon MWNT singles yarn, (b) a twofold MWNT carbon nanotube yarn, and(c) a PVA-infiltrated MWNT singles yarn.

FIG. 6 shows SEM micrographs of an overhand knot in a twofold carbonmulti-wall nanotube yarn that has about the same diameter as the yarn.

FIG. 7 shows the hysteretic stress-strain curves (1%/min strain rate)observed on unloading and reloading a twofold carbon MWNT yarn over a1.5% strain range after prior mechanical conditioning.

FIG. 8 shows, for the stress-strain loops in FIG. 7, the energy loss percycle versus the initial strain on unloading for cycles of 1.5% instrain (squares) and a 0.5% in strain (circles).

FIG. 9 shows the effective Young's modulus of a twofold carbon MWNT yarnas a function of the stage of the hysteresis cycle, wherein theeffective yarn modulii calculated for the stress-strain loops shown inFIG. 7 are plotted versus total tensile strain, and wherein the circlesand squares are the effective modulii for the beginning and end ofunloading, respectively, and the diamonds and triangles are those forthe beginning and end of reloading, respectively.

FIG. 10 shows the relationship between percent change in diameter andpercent change in length during stretching a twofold MWNT yarn (top) anda singles MWNT yarn (bottom), wherein the symbols used are: open circles(initial stretch), solid diamonds (the first stress decrease), solidclosed circles (second stress increase), solid triangles (second stressdecrease), and solid squares (stress increase until yarn rupture),wherein the curves are guides for the eye.

FIG. 11 shows a photograph of a spun carbon MWNT ribbon that ishelically wrapped on a 1 mm diameter hollow capillary tube, wherein thehigh transparency (resulting from a wrap thickness that provides a lowlayer resistively) is indicated by the legibility of a ¾ point line thatis on paper sheet behind the nanotube ribbon-wrapped capillary tube.

FIG. 12 shows an SEM micrograph of an overhand knot in a singles MWNTtwisted yarn, wherein the relative yarn dimensions at places removedfrom the knot, at the knot entrance and exit, and in the body of theknot provide regional density differences that can be used for selectiveregion infiltration and reaction, and wherein the pictured straynanotubes that migrate from the knot and other regions of the knot canoptionally be removed chemically (such as by passing the yarn through anopen flame), and if desired for applications like electron fieldemission, the density of these stray nanotubes can be selectivelyincreased in different regions of the yarn by mechanical treatments orchemical treatments, including chemical treatments that result innanofiber rupture.

FIG. 13 shows an SEM micrograph of an overhand knot tied in one twofoldMWNT yarn, so that the knot includes a second twofold MWNT yarn, whereinsuch intersections between initially independent yarns can be used aselectrical junctions and as microfluidic junctions, and wherein thedegree of interaction between the two yarns (electrical contactresistance and resistance to microfluidic mixing) can be varied bytightening the knot.

FIG. 14 shows an SEM micrograph of CVD-produced coiled carbon nanofibersthat are useful for producing highly extensible nanofiber yarns.

FIG. 15 shows an SEM micrograph of a section of a 280 micron high forestof crimped and aligned nanofibers that was produced by a CVD process.

FIG. 16 is a schematic picture of a textile weave that provides dockingsites for functional devices (such as substrate-released electronicchips).

FIG. 17 is a photograph of a spun MWNT ribbon that is deposited on oneside of a glass microscope slide to make a transparent conducting sheet,wherein the printing of a logo is on a sheet of white paper that isbeneath the electrically conducting layer.

FIG. 18 is a picture of a twofold twisted MWNT yarn that has beenelectrically heated to incandescence in an inert atmosphere chamber.

FIG. 19 is a line drawing illustrating a preferred spinning system forproducing MWNT yarns in accordance with some embodiments of the presentinvention, wherein the drawing shows how the yarn can be formed withoutthe need for rings, travelers, or caps that would significantly increasethe tension in the yarn.

FIG. 20 is a schematic drawing showing details of the substrate holderthat in this case optionally uses 6 substrates with nanotubes optionallycoated on both sides of a substrate.

FIG. 21 is a photograph of a self-supporting 3.4-cm-wide meter-long MWNTsheet that has been hand drawn from a nanotube forest at an average rateof 1 m/min, wherein sheet transparency is illustrated by the visibilityof the NanoTech Institute logo that is behind the MWNT sheet.

FIG. 22 is a Scanning electron microscopy (SEM) image, at a 35° anglewith respect the forest plane, capturing a MWNT forest being drawn intoa sheet.

FIG. 23 is a SEM micrograph showing the cooperative 90° rotation ofMWNTs in a forest to form a strong carbon nanotube sheet.

FIG. 24 shows sheet resistance R(T) measured in vacuum, normalized withrespect to R(300 K), versus temperature. The inset shows the nearlyidentical temperature dependence of sheet resistance in the drawdirection for forest-drawn sheets before (solid rectangles) and afterdensification (open circles), in the orthogonal direction for thedensified forest-drawn sheet (diamonds), and for a filtration-fabricatedsheet of forest-grown MWNTs (solid circles).

FIG. 25 shows optical transmittance versus wavelength for a single MWNTsheet, before and after densification, for both polarized andunpolarized light, where the arrow points from the data for theundensified sample to that for the densified sample.

FIG. 26 shows noise power density (measured in air for 10 mA biasing)versus frequency for a densified forest-drawn MWNT sheet (open circles),compared with that for ordinary filtration-produced MWNT sheets (solidcircles) and SWNT sheets (solid rectangles) having the same 40 ohmresistance. The dashed lines are data fits for a 1/f^(α) dependence onfrequency (f), where α is 0.98±0.04, 0.97±0.02, and 1.20±0.02,respectively. The lower-limit noise power at temperature T (the productof 4k_(B)T and the sample resistance R, where k_(B) is Boltzmann'sconstant) is indicated by the horizontal dotted line.

FIG. 27 is a SEM micrograph of a two-dimensionally re-reinforcedstructure fabricated by overlaying four nanotube sheets with a 45° shiftin orientation between successive sheets.

FIG. 28 is a SEM image showing the branching of fibrils within adensified solid-state fabricated MWNT sheet.

FIG. 29 shows mechanical property measurements for as-drawn MWNT sheetscut from one original sheet and stacked together so that they have acommon nanotube orientation direction. (A) Engineering stress versusstrain, showing surprisingly small variation in maximum stress forsamples containing different numbers of sheets that are stackedtogether. (B) The maximum force and the corresponding strain as afunction of number of stacked sheets for the sample runs of (A).

FIG. 30 is a photograph showing an as-drawn nanotube sheets supportingmillimeter scale droplets of water, orange juice, and grape juice wherethe mass of the millimeter size droplets is up to 50,000 times that ofthe contacting nanotube sheets.

FIG. 31 is a photograph showing a free-standing, undensified MWNT sheet(16 mm×23 mm) used as a planar incandescent light source that emitspolarized radiation, wherein the background color for the unheated sheet(A) and the incandescent sheet (B) differs because of reflectedincandescent light from a white paper sheet that is behind the lightsource.

FIG. 32 shows spectral radiance in directions parallel to (∥) andperpendicular to (⊥) the draw direction of an as-drawn, undensified MWNTsheet after an added inelastic stretch in the initial draw direction of2.5%. The inset shows this data on a semi-logarithmic scale. Underlyingsolid lines (largely obscured by coincidence with the data points) aredata fits assuming black body radiation with T=1410 K.

FIG. 33 is a photograph of two 5-mm thick Plexiglas plates that havebeen welded together by microwave-induced heating of an as-drawn MWNTsheet that was sandwiched between these plates, wherein this weldingprocess maintains the electrical conductivity, nanotube orientation, andtransparency of the nanotube sheet.

FIG. 34 provides photographs of an electronically conducting andmicrowave absorbing appliqué comprising an undensified MWNT sheetsattached to transparent adhesive tape (Scotch Packaging Tape from 3MCorporation), wherein the transparency of the appliqué is indicated bythe visibility of the logo and the “UTD” printed on a paper sheetunderneath the appliqués. The appliqué is folded in the bottom picture(and held together using a paper clip) for experiments that show thatthe sheet resistance is little effected by the folding.

FIG. 35 shows that 100% elongation of a silicone rubber sheet withattached MWNT sheet causes little change in the sheet resistance of theMWNT sheet (uncorrected for geometry change in going from stretched tocontracted state).

FIG. 36 is a photograph of an organic light emitting diode (OLED) thatuses a transparent solid-state-fabricated MWNT sheet as thehole-injecting electrode.

FIG. 37 shows that MWNTs in a MWNT sheet on one substrate can bemechanically transferred to produce a printed image (“UTD NanoTech”) onanother substrate. This transfer occurs without substantial loss ofnanotube orientation. The picture on the left shows the MWNT sheetattached to the substrate (non-porous paper) after the transfer processand the picture on the right shows ordinary writing paper with thetransferred image.

FIG. 38 schematically illustrates a process of utilizing one motor tosimultaneously and independently vary twisting and winding rates foryarns. The process imposes minimal tension on the spun yarns, which isespecially useful when the yarn diameter is very small, when lowstrength yarns are being processes prior to subsequent strengthenhancement, or when low strength yarns having high elasticdeformability are needed.

FIG. 39 shows an optical micrograph of a multiwalled nanotube yarn woundhelically on a bobbin (a 5 mm diameter plastic tube) during twist-basedspinning of yarn from a carbon nanotube forest.

FIG. 40 shows two optical micrographs of a CNT-wool composite yarn inwhich CNT fibers and wool fibers were introduced during twist spinning.

FIG. 41 The high degree of nanotube orientation in the nanotube sheet isdemonstrated by this Raman data for an as-drawn four-sheet stack inwhich all sheets have the same orientation. A VV configuration (parallelpolarization for incident light and Raman signal) was used, withpolarization parallel to (∥) or perpendicular to (⊥) the draw directionof the nanotube sheets.

FIG. 42 is an optical micrograph showing a two-ply MWNT yarn (comprisedof 12 μm diameter singles yarns) that has been inserted in aconventional fabric comprising 40 μm diameter melt-spun filaments.

FIG. 43 schematically illustrates a rotor spinner that convertsnanofibers, such as carbon nanotubes, into twisted yarn.

FIG. 44 schematically illustrates an apparatus for densifying carbonnanotube yarns using a spinneret that introduces false twist, meaningthe absence of net twist. An optional method for additionaldensification and inserting additives (a syringe pump) is also pictured.

FIG. 45 provides details for the spinneret of FIG. 44 for introducingfalse twist.

FIG. 46 schematically illustrates a spinning apparatus in which afalse-twist spinneret is used for the densification and strengthenhancement for a nanotube yarn before the introduction of net twist(also called real twist).

FIG. 47 shows the dependence of electrical resistance upon the twistlevel (in turns/meter) for a solid-state spun MWNT yarn.

FIG. 48 shows the dependence of ultimate tensile stress as a function ofyarn helix angle (with respect to the yarn direction) for a solid-statespun MWNT yarn, where the yarn samples corresponding to open circles aretwist spun without any prior treatment and the yarn samplescorresponding to open squares where initially densified by liquidinfiltration and liquid evaporation in order to avoid too severe adrop-off in nanotube yarn strength with decreasing twist angle.

FIG. 49 shows the dependence of failure strain as a function of yarntwist angle (with respect to the yarn direction) for a solid-state spunMWNT yarn samples of FIG. 48.

FIG. 50 shows the dependence of ultimate tensile stress as a function ofyarn diameter for low and high twist yarns.

FIG. 51 shows the dependence of failure strain as a function of yarndiameter for low and high twist yarns.

FIG. 52 compares SEM images of (a) Yarn A: with a 26000 turns/mclockwise twist and (b) Yarn B: a 26000 turns/m clockwise twist wasintroduced first and then the same twist was introduced anticlockwise torelease the twist.

FIG. 53 shows a process in which nanotube sheets can be drawn, attachedto a substrate film, densified by immersion in a liquid and evaporationof this liquid, and then wound onto a mandrel.

FIG. 54 shows a process in which nanotube sheets can be drawn, attachedto a substrate film, densified using exposure to a vapor, and thencollected on a mandrel.

FIG. 55 shows a process for laminating a nanotube sheet between films.

FIG. 56 compares SEM micrographs of the growth substrates for spinableand non-spinable nanotube forests (after removal of the nanotubes)wherein the small diameter pits on the growth substrate correspond tothe growth site of a MWNT.

FIG. 57 provides SEM micrographs showing that the PVA infiltration hasnot disrupted the twist-based structure of a MWNT singles yarn.

FIG. 58 is a SEM micrograph showing about twenty MWNT singles yarns thathave been plied together to make a twenty-fold yarn having a diameterabout equal to that of a human hair.

FIG. 59 A-C schematically illustrate components and successive stagesduring the fabrication of a matrix addressable bolometer. A) depicts afree-standing nanofiber sheet conductor, which has highly anisotropicelectrical and thermal conductivities as a result of nanofiberorientation; B) depicts a frame comprising arrays of metal electrodepads and having a rectangular central opening over which two nanofibersheet conductors (of the type shown in A) can be suspended from oppositeframe sides so that the orientation direction of the nanofiber sheetconductors are orthogonal. The metallic electrode pads are covered witha thin film of temperature sensitive material. C) depicts the twoorthogonally aligned nanofiber sheets attached to opposite sides of theframe in accordance with some embodiments of the present invention.

FIG. 59 D shows an example of a single sheet bolometer that uses thinwires (5911) of iron and of constantan as a sensitive thermocouple.

FIG. 60 schematically illustrates an anisotropic resistor with lowtemperature coefficient of resistivity, which can deposited on a flatinsulating substrate or rolled on an insulating cylindrical substrate toprovide a required resistance determined according to the utilizednumber of turns.

FIG. 61 shows schematically aligned nanofiber sheets in the architectureof a transparent electromagnetic (EM) shield, which can advantageouslyuse the flexibility, electrical conductivity, transparency,radiofrequency and microwave frequency absorption, and dichrosimobtainable for solid-state drawn nanofiber sheets.

FIG. 62 A schematically illustrates a gas sensor using a carbon MWNTnanotube sheet, whose sensitivity is increased by the use of a depositedlayer of SWNTs. While the MWNT sheet is herein schematically representedusing a series of parallel lines it should be recognized that there is adegree of lateral connectivity for the MWNT sheets and this degree oflateral connectivity both increases sheet mechanical robustness anddecreases sheet anisotropy. FIG. 62 B demonstrates the feasibility ofthe device concept by demonstrating that a large change in resistanceresults when a SWNT sheet is exposed to either benzene of alcohol vapor.

FIG. 63 schematically illustrates a transparent antenna, made oforiented nanofiber sheets that are laminated on an optionally flexibleor elastomeric insulating substrate

FIG. 64 schematically illustrates a nanofiber-sheet-based heat exchangerfor dissipation of excessive heat from microelectronic chips. Thenanotube sheet is connected by lamination to the heat sink, shown as acopper plate.

FIGS. 65 A-D are SEM images depicting different types of carbon nanotubeyarn cathodes that were fabricated by using an initial draw process froma carbon nanotube forest: (A) a twisted singles yarn, (B) a knottedtwo-ply yarn, (C) multiple yarns that are knotted together, and (D) asingles yarn helically wrapped on a glass capillary.

FIGS. 66 A and 66 B schematically illustrate (A) the geometries oflateral planar cathode and (B) a vertical single-end cathode for fieldemission from MWNT twisted yarns, respectively.

FIG. 67 is a typical current-voltage (I-V) plot of field emission from aMWNT yarn in the vertical, single-end geometry. A short (1 ms) highvoltage pulse (2 kV) was pre-applied to raise the yarn vertically.

FIG. 68 shows the spectrum of light emitted from a single twisted MWNTyarn end, and the fit of this spectrum to Planck's black body radiationlaw (solid line). At high currents, this light emission from thenanotube yarn accompanies electron emission. The Inset shows aphotograph of the incandescent light source that appears on the tip ofthe carbon nanotube yarn when using high current.

FIG. 69 shows current versus applied voltage for electron emission froma side of a twisted multiwall carbon nanotube yarn. A decrease in onsetvoltage as a result of hysteresis behavior during the first voltagecycle can be seen, indicating that this initial cycle improves electronemission.

FIGS. 70 A and 70 B are phosphor-screen images showing the emissionuniformity of the carbon multiwall nanotube yarns in lateral geometry(i.e., emission lateral to a side of a nanotube yarn). Images were takenusing a one millimeter gap between the cathode and the phosphor screenanode. The voltages applied were negative pulses of 1.5 kV and 3 kV forthe A and B images, respectively.

FIG. 71 is phosphor screen image showing a prototype of a patternedalpha numeric display based on electron emission from multiwallednanotube yarns in a flat, patterned geometry. Images were taken with a 1mm gap between the cathode and the phosphor screen anode. Negative 3 kVvoltage pulses were applied to the cathode. The repetition rate of thepulses was 1 kHz and duty cycle was 1%.

FIG. 72 schematically illustrates a twisted nanofiber yarn, inaccordance with some embodiments of the present invention, whereinelectron field emission occurs predominately from the sides of thenanofiber yarn. Nanotubes and nanotube bundles extend laterally from theyarn sides and thereby provide amplified field emission throughconcentration of field lines. Such lateral nanotubes and nanotubebundles concentrate the electric field lines (shown as arrows comingfrom the anode), thereby enhancing field emission.

FIG. 73 schematically illustrates field emission from the end of anelectrically conducting nanofiber yarn, enhanced by untwisting of an endof the twisted nanofiber yarn in high electric field, in accordance withsome embodiments of the present invention. This type of nanofiberyarn-end emission can be particularly suitable for use as a point typeelectron source.

FIGS. 74 A and B show SEM micrographs, at two different magnifications(A and B), of the unwrapping of a yarn during field emission, whichcreates a “hairy” yarn with multiple nanofiber end tips, such tips beingparticularly suitable for enhanced field emission of electrons.

FIG. 74 C schematically illustrates formation of a hairy nanofiber yarnas a result of electric field effect on yarn regions that are closest tothe anode. The illustrated nanofiber yarn is spirally wrapped around awire or cylindrical capillary.

FIGS. 75 A-D are images on phosphorescent screen depicting lightemission from a nanofiber-yarn-based cathode in a lateral geometry bothwithout (A and B) and with (C-D) knots. In the pictured case, theknotted part of the yarn cathode shows suppressed of field-inducedemission, which can be used for patterning electron emission fromnanofiber yarn cathodes.

FIGS. 76 A and 76 B are current versus voltage (A) and current versustime (B) curves of self-improving yarn cold cathodes, showing anincrease of the current density and a lowering of the threshold andoperation voltage upon increasing the time of operation.

FIG. 77 illustrates the concept of using a textile for supportingelectron emitting nanofiber yarns, wherein two electrically conductingnanofiber yarns are woven orthogonally into an otherwise substantiallynon-electron-emitting textile. This insulating textile (or onecomprising electrically conducting nanofiber-free wires) provides aflexible support for the electron emitting nanofibers and enables theirplacement in a patterned manner within the textile for electron emissionpurposes. The textile and any of the components in the textile can servefor dissipation of generated heat, and the current density can be tunedby varying the density and topology of the woven structure.

FIG. 78 is a picture showing an operating phosphorescent lamp in whichthe electron emitting element is a twisted carbon MWNT yarn. Thisnanofiber yarn cathode is wrapped about a copper wire located at centerof a glass cylinder.

FIG. 79 schematically illustrates a transparent cold cathode electronemitter comprising a carbon nanotube sheet (7901) supported by either atransparent insulating substrate (7902) or a substrate coated with atransparent electrically conducting film. For the case where thecontacting substrate is insulating, the transparent nanotube sheet iscontacted with an electrical contact material, which can be anelectrically conducting tape (7903).

FIG. 80 schematically depicts two electron emission pathways fromnanotube sheets: (top portion) electron emission due to fieldenhancement at the ends and sides of nanofibers and (bottom portion)electron emission from tips (free ends extended from the sheet) and fromthe sides of nanofibers within the sheet.

FIG. 81 schematically depicts a non-transparent electron emitter used inconventional configuration for a phosphorescent display or lamp, whereinthe cathode (8102 electron emitter) is on the back side of the displayor lamp and light is exclusively provided in the frontal direction fromthe phosphorescent screen (8105) that is on the front side of thedisplay or lamp.

FIG. 82 schematically depicts an electron emitter used in anotherconventional configuration for a phosphorescent display, wherein thecathode (8202 electron emitter) is on the back side of the display(i.e., behind the phosphorescent screen 8205) and the charge collectoris a transparent ITO film (8204). In this geometry some light isradiated backwards. Resulting reflection from the back part of thedisplay creates various problems, such as decrease of display contrastand resolution.

FIG. 83 schematically illustrates a display architecture of the presentinvention where a transparent nanofiber sheet electron emitter (8303) ison the front of the display, light emitted by the phosphor screen (8305)is reflected by the back anode plate (8306) and reaches the viewer afterpassing through the transparent nanofiber sheet cathode (8303).

FIG. 84 schematically illustrates how a transparent nanotube sheetelectron-emitting cathode can be used for generating an effectiveback-light source for a conventional liquid crystal display (LCD). Theback source light is polarized (which is desirable for LCD operation),since the transparent cold cathode acts as a polarizer due to theexistence of highly oriented nanotubes in the nanotube sheet.

FIG. 85 depicts a polymeric light-emitting diode (PLED) which uses adensified transparent carbon nanotube (CNT) sheet as the anode, in placeof the typically used indium tin oxide (ITO). Due to the flexibility ofthe carbon nanotube sheets, this PLED can be highly flexible if thesubstrate is a flexible.

FIG. 86 depicts an organic light-emitting diode (OLED) which uses acarbon nanotube sheet as an anode. Similar to the PLED, this device hasthe active low molecular weight organic layers deposited on top of adensified transparent nanotube sheet. Due to the flexibility of thecarbon nanotube sheets, this PLED can be highly flexible if thesubstrate is a flexible.

FIG. 87 depicts a PLED with a bottom-up construction, starting with thecathode layer and subsequent deposition of the organic/polymericfunctional layers. The final layer, the carbon nanotube sheet (anode),is placed on the device by means of stamping from another substrate, orby laying a free-standing nanotube sheet over the device.

FIG. 88 depicts the bottom-up structure described in FIG. 87 on asilicon wafer for active matrix OLED. The silicon wafer may containaluminum/calcium contact pads or transistors. The polymeric layers aredeposited first, followed by the nanotube sheet anode on the top.

FIG. 89 depicts a transparent PLED which uses a carbon nanotube sheet asboth the anode and the cathode. As a result, both electrodes and thedevice itself are transparent. The device can be built on aflexible/elastomeric substrate, thereby realizing the ultimate goal of aflexible transparent display.

FIG. 90 illustrates a solar cell or photodetector based on carbonnanotube ribbons as a top transparent conducting electrode, inaccordance with some embodiments of the present invention.

FIG. 91 illustrates a solar cell or photodetector based on carbonnanotube ribbons as a bottom transparent conducting electrode, inaccordance with some embodiments of the present invention. A SEM imageof part of the carbon nanotube ribbon electrode is also shown on theleft, after 90° rotation from alignment in the device.

FIG. 92 illustrates a transparent solar cell or photodetector based oncarbon nanotube ribbons as top and bottom transparent conductingelectrodes, in accordance with some embodiments of the presentinvention.

FIG. 93 illustrates a tandem solar cell or photodetector based on carbonnanotube ribbons as a upper transparent conducting electrodes.

FIG. 94 shows the examples of spectral sensitivity of polymeric solarcells, demonstrating the additional functionality of carbon nanotubecharge collectors for enhancement of light absorption and chargegeneration in UV and IR spectral bands. Curve 1 corresponds to ITOanode, curves 2-4 to carbon nanotube sheet anode, and curve 5 to carbonnanotube sheet anode coated with very thin Au/Pd layer.

FIG. 95 depicts, schematically, a plastic solar cell based on conjugatedpolymer/fullerene photoactive bulk donor-acceptor heterojunction withtransparent nanotube sheet anode (hole collector) and polymer/nanotubewires nanointegrated as photoactive electron collectors. Insets show thehole transfer from conducting polymer chains into the nanotubes of thetransparent nanotube sheet anode, and also an electron transfer, uponexciton dissociation in a polymer/single wall nanotube system.

FIG. 96 schematically depicts conjugated polymer/nanotube compositeintegrated on the nanoscale inside 100 nm scale (on average) poreswithin the porous structure of a transparent nanotube sheet. Holesphotogenerated in the filled pores are collected on sheet nanotubeswithin the charge collection length of the polymer (about 100 nm), asshown by arrows.

FIG. 97 shows current versus voltage curves of a polarization-sensitivephotocell that utilizes a transparent oriented carbon nanotube sheetanode.

FIG. 98 shows an SEM image of a carbon nanotube sheet in which a verythin film (5 nm) of Au/Pd was sputtered on top of the nanotubes. An SEMimage of an uncoated nanotube sheet is shown for comparison.

FIG. 99 schematically illustrates a transparent organic field effecttransistor (OFET) with transparent gate, source and drain electrodesmade of carbon nanotube sheets and the active channel of organic orpolymeric semiconductor. The transparency of this device enablesphotomodulation of source-drain current, which is useful for opticalchip-to-chip information transfer.

FIG. 100 is a schematic diagram showing the basic structure of a priorart dye-sensitized solar cell, in which nanofiber sheets can be used asan anode.

FIG. 101 schematically illustrates novel architecture for adye-sensitized solar cell (DSC), wherein a conventional anode of a DSCis replaced by transparent nanofiber sheets, ribbons, or yarns ofpresent invention embodiments. The cathode can also use transparentnanofiber sheets, ribbons, or yarns (instead of traditional ITO) coatedby nanoporous titania.

FIG. 102 shows an SEM image of single wall carbon nanotubes depositedfrom a liquid onto a transparent sheet of multiwall carbon nanotubesmade using a solid-state sheet fabrication process or inventionembodiments.

FIG. 103 illustrates a multijunction solar cell (tandem solar cell), inwhich the transparent top electrode (10301), and one or more transparentinner interconnect electrode sheets enable increased energy harvestingefficiency through expanding the harvested light to a broader region ofthe solar spectrum.

FIG. 104 schematically illustrates a carbon nanotube yarn based fuelcell.

FIG. 105 schematically illustrates a carbon nanotube yarn based heatpipe.

FIG. 106 schematically illustrates an apparatus for continuouslyspinning carbon nanotubes and other nanofibers into twisted yarns,wherein two motors are used to insert twist and wind the yarn on abobbin and the relative rotation rates of these motors determines theinserted twist per yarn length.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Invention embodiments described herein provide novel fabricationmethods, compositions of matter, and applications of nanofiber yarns,ribbons, and sheets having quite useful properties. For example, carbonnanotube yarns of the invention embodiments provide the following uniqueproperties and unique property combinations: (1) toughness comparable tothat of fibers used for bullet proof vests, (2) resistance to failure atknots (contrasted with the sensitivity to knotting for the Kevlar® andSpectra® fibers used for antiballistic vests), (3) high electrical andthermal conductivities, (4) high reversible absorption of energy, (5) upto 13% strain-to-failure compared with the few percent strain-to-failureof other fibers with similar toughness, (6) very high resistance tocreep, (7) retention of strength even when heated in air at 450° C. forone hour, and (8) very high radiation and UV resistance, even whenirradiated in air.

Moreover, the Inventors (i.e., Applicants) show that these nanotubeyarns can be spun as one micron diameter yarns (or either lower or muchhigher diameter yarns), and plied at will to make twofold, fourfold, andhigher folded yarns. Additionally, the inventor's show that novel yarnshaving the above-described properties can be spun using either carbonSWNTs or carbon MWNTs, the latter being much less expensive to producethan the former.

Invention embodiments also provide for the fabrication of nanofibersheets having arbitrarily large widths at commercially useful rates.These sheets are optically transparent and have a higher gravimetricstrength than the strongest steel sheet and the Mylar® and Kapton®sheets presently used because of their high gravimetric strength forultra-light air vehicles.

Importantly, the inventors also teach how this technology can be used toproduce various yarns, sheets, and ribbons of diverse nanofibers, andhow produced yarns, sheets, and ribbons of these nanofibers can beapplied.

For the purpose of most efficiently and clearly describing theembodiments of this invention, a nanofiber is herein defined as a fiberor ribbon having a largest thickness normal to the fiber axis of lessthan 100 nm.

Since the smallest possible nanofibers for a particular system can formbundles and aggregates of bundles (which can potentially also fall underthe above definition of nanofibers), the Inventors hereby define thenanofibers described herein to be the smallest diameter nanofibers whosediscrete nature is importantly relevant either for the assembly ofpre-processing arrays or the structure of fabricated sheets, ribbons, oryarns.

Also, unless specifically otherwise indicated, no differentiation ismade between the terms knitted, braided, and woven. The reason forignoring differences in these terms herein is that statements made aboutany one of these terms typically applies generically to all of theseterms and like terms. Also, the terms two-fold and two-ply, and liketerms are used equivalently for plied (i.e., folded) yarns.

The challenge of spinning nanofibers into twisted yarns of thisinvention is in downscaling prior-art technologies of usual spinningabout a thousand fold to provide the special geometries in which twistcan be successfully utilized. Optionally and preferably the maximumtotal applied twist in one direction per unit fiber length for a twistedyarn of diameter D is at least approximately 0.06/D turns and asignificant component of the nanofibers have (i) a maximum width of lessthan approximately 500 nm, (ii) a minimum length-to-width ratio of atleast approximately 100, and (iii) a ratio of nanofiber length to yarncircumference greater than approximately 5.

For the purpose of fabricating nanotube sheets and ribbons bysolid-state draw processes, optionally and preferably a significantcomponent of the nanofibers have (i) a maximum width of less thanapproximately 500 nm and a (ii) a minimum length-to-width ratio of atleast approximately 100.

The net twist is defined as the overall twist introduced during allprocessing from an initial fiber assembly to the product used forapplications. Processes that result in untwisting are included in theevaluation of net twist. Optionally and preferably for twisted yarns,the maximum total twist in one direction (uncompensated by the possibleoccurrence of twist in an opposite direction) is at least 0.06/D.Optionally and preferably, the net twist (defined as compensated byapplied yarn twists in opposite directions) for the nanofiber yarn canvary from negligible to at least approximately 0.12/D turns for someapplications and at least 0.18/D for other applications. Especially forthe purpose of applications where high deformability without rupture andlow yarn strength is needed (such as for yarn actuators where theactuating material is predominately a material that is imbibed into theyarn), the net yarn twist is preferably above 0.18/D.

For the purposes of this invention, a false twist is herein defined as atwist in one direction that is followed by an essentially equal twist inthe opposite direction, so that the net twist is essentially zero. Thedefinition is herein applied whether the net twist of near zero isintroduced by simply applying a twist at an intermediate position in ayarn (so that twist is introduced on one side of the twist position andautomatically substantially removed on the opposite side of the twistposition) or by first twisting at the end of a yarn and then releasingthe original twist by applying an equal and opposite twist to the sameyarn end.

The Inventors differentiate between ribbons and ribbon shaped yarns(also called yarns) by defining a ribbon as having a width of at least amillimeter.

For the purpose of draw-based fabrication processes the inventors heredefine pre-primary and primary nanofiber assemblies. A pre-primaryassembly of nanofibers is an assembly of at least approximately parallelnanofibers (i.e., oriented nanofibers) that undergoes a substantialchange in nanofiber orientation direction during the process of makingan oriented nanofiber yarn, a twisted nanofiber yarn (including a falsetwisted yarn having little net twist), a nanofiber ribbon, or ananofiber sheet. A primary assembly of nanofibers is an orientednanofiber array or a nanofiber array that is converging on anorientation direction, wherein the draw direction for yarn, sheet, orribbon formation is either the direction of nanofiber orientation or thedirection of nanotube orientation that is being converged upon.

A solid-state fabrication process is one that can be practiced withoutthe required presence of a liquid during nanofiber yarn, ribbon, orsheet formation.

Unless the needed nanofiber properties are obviously not thoseobtainable for carbon nanotubes, carbon nanotubes are nanofibers thatare included in the group of optionally preferred nanofibers for theembodiments of this invention. Also, the fabrication method employed forconverting a nanofiber array into a sheet, ribbon, or yarns ispreferably by solid-state fabrication methods of invention embodiments.

Also, a yarn made by plying at least one singles yarn is understood tocomprise a singles yarn.

So that invention embodiments can be further understood, the inventorsherein use the term knot for both mathematical knots and knots calledunknots, since unknots can be inexpensively tied without using the endsof a robe or yarn. Useful examples of unknots are slip knots.

Unknots are important because they can be economically produced, such asin conventional weaving processes, and because their untying unknotsthat are slip knots (by applying stress to yarn ends) is a useful way toincrease energy dissipation before failure for a given weight nanofiberyarn (i.e., yarn toughness).

1. Forest-Based Nanofiber Fabrication Invention Embodiments

(a) Twist-Based Yarn Spinning from Nanofiber Forests

One preferred process of this invention involves twist-based spinning ofcarbon nanotubes from a nanotube forest, so called because the nanotubesgrow from a substrate like approximately parallel trees and have closeto the same height. The nature of the nanofiber forest used for spinningis critically important, and will be elaborated on in another section.Most nanofiber forests are unsuitable for twisted yarn, ribbon, or sheetproduction. Other nanofiber forests yield yarns or ribbons which are fartoo weak to be useful for most applications. For example, Jiang et al.have described (in Nature 419, 801 (2002) and in U.S. Patent ApplicationPublication No. 20040053780 (Mar. 18, 2004)) the production of very weakuntwisted yarns from nanotube forest that they grow. These yarns areweak likely both because of the low performance of the utilized carbonnanotube forest and the lack of realization that twist processes can bedownscaled to nanofibers to provide strength increases.

In some important embodiments the yarns of the present invention aresimultaneously twisted while being drawn from a nanotube forest. FIG. 1shows an arrangement used in the laboratory for accomplishing this twistprocess during drawing, and FIGS. 2 A and 2B are SEM pictures showingnanotube assembly into yarn during the spinning process, in which thenanotubes are simultaneously drawn from the nanotube forest and twisted.Element 101 in FIG. 1 is a nanotube forest, prepared as described inExample 1. Element 102 is the silicon growth substrate, element 103 is aribbon drawn from the forest, and element 104 is the nanotube yarnresulting from twisting this ribbon, which is wedge-shaped when viewedin 3-D and has a maximum wedge thickness of about equal to the height ofthe nanotube forest. Element 104 is often referred to in conventionaltextile processing as the spinning triangle. The overlapping images ofboth the nanotube wedge and yarn are a result of reflection in themirror-like silicon substrate. Element 105 is the end of a miniaturewooden spindle about which the nanotube fiber has been wrapped. Element106 is adhesive tape that attaches this wooden spindle to the rotatingrod of motor 107.

The direction of drawing is very nearly orthogonal to the originalnanotube direction and parallel to the silicon substrate in FIG. 1.Nevertheless, this spinning process is sufficiently robust that theangle between the initial nanotube direction and the draw direction canbe decreased from 90° to almost 0°. Although this spinning process isamenable to automation for spinning continuous yarns, Example 2describes yarns that were prepared by hand drawing from a nanotubeforest while they were twisted using a variable speed motor operated at˜2000 rpm. The diameter of the twisted yarns ranged from below onemicron to above 10 microns, and depended upon the sidewall area on thenanotube forest from which the MWNT yarn was drawn. The obtainedcombination of yarn diameters that are hundreds of times smaller thanthe nanotube length (about 300 μm) and high twist (about 80,000 turn/m)resulted in yarns having quite attractive properties.

For twist-based nanofiber draw and other nanofiber draw processes ofinvention embodiments for yarn spinning, it is preferred that nanofibersare simultaneously pulled from essentially the full height of thenanofiber forest sidewall.

The Inventors have found that achieving very useful properties from suchtwist spinning of nanoscale fibers requires that a number of conditionsare optimally satisfied, and these conditions provide the basis fornumerous preferred embodiments. Such conditions are described below.

First, a significant component of the nanofibers should optionally andpreferably have a maximum thickness of less than approximately 500 nm.For circular nanofibers, this maximum thickness corresponds to thenanofiber diameter (which is the nanofiber width if the nanofiber is ananoribbon). More optionally and preferably, a significant component ofsaid nanofibers should have a maximum thickness of less thanapproximately 100 nm. Most optionally and preferably, a significantcomponent of said nanofibers should have a maximum thickness of lessthan approximately 30 nm. By significant component, the Inventors mean acomponent of the nanofiber thickness distribution that is sufficientlyprevalent to significantly affect yarn properties.

Second, the nanofibers optionally and preferably should have a minimumaspect ratio, i.e., ratio of nanotube length to diameter, of at leastapproximately 100 at the thinnest lateral section. More preferably, thenanofibers should have a minimum length-to-thickness ratio in thethinnest lateral direction of at least approximately 1000. Mostpreferably, the majority weight fraction component of the nanofibersshould have an aspect ratio in the thinnest lateral section of at leastapproximately 10000.

Third, optionally and preferably the nanofibers in the yarns should havea minimum ratio of nanofiber length to yarn circumference that isgreater than approximately 5. More preferably, the nanofibers in theyarns have a minimum ratio of nanofiber length to yarn circumferencethat is greater than approximately 20. Most preferably the major weightcomponent of the nanofibers has a ratio of nanofiber length to yarncircumference of greater than approximately 50

Fourth, the maximum applied twist per yarn length, uncompensated bypossibly applied twist in an opposite direction, for a twisted yarn ofdiameter D is optionally and preferably at least approximately 0.06/Dturns. Optionally and more preferably for some applications, the maximumapplied twist for a twisted yarn of diameter D is at least approximately0.12/D turns. Optionally and most preferably for some applications, themaximum applied twist for a twisted yarn of diameter D is at leastapproximately 0.18/D turns. Preferably for some applications, themaximum applied twist is preferably above 0.06/D turns and below 0.12/D.

The weight-averaged nanofiber length in the yarn is optionally andpreferably at least approximately 2 times the inverse of the yarn twist(measured in turns per yarn length).

Additionally, it is optionally preferable that at least a 20% of thetotal weight fraction of the nanofibers in the yarns migrate from nearto the yarn surface to deep in the yarn interior and return to close tothe yarn surface in a distance that is less than approximately 50% ofthe nanofiber length. More preferably, the major weight fraction ofnanofibers in the yarns migrate from near to the yarn surface to deep inthe yarn interior and return to close to the yarn surface in a distancethat is less than approximately 20% of the nanofiber length.

The twist-based spinning process for nanofiber yarns is preferablyaccomplished by arranging nanofibers in approximately aligned arrays orin an array that is converging towards alignment so as to provide aprimary assembly. A primary assembly is above defined as one in whichthe nanotubes are either approximately arrayed parallel to a drawdirection or are converging on such alignment. Hence, a nanotubeassembly can possibly change from a primary assembly to the abovedefined pre-primary assembly—depending upon the direction of draw.

This primary assembly can optionally be formed from a precursorassembly, such as a nanotube forest, which can be either a primaryassembly or a pre-primary assembly, depending upon the direction ofdraw. Nanofibers converging towards alignment can be formed by drawingthese nanofibers from a nanotube forest. This nanofiber forest issuitable for formation of the primary assembly, or as a primaryassembly, can be on a planar or non-planar substrate and the nanofibersin the forest can either be deposited over substantially the entiresubstrate surface or only on part of the surface. Also, different typesof nanotubes can be in different regions of the surface, or they can bemixed in the same forest regions.

For convenience in facilitating nanotube yarn spinning and ribbon andsheet draw, the minimum radius for the forest-occupied-area of a curvedforest substrate is optionally over ten times the maximum height of theforest.

The twist process to make yarn can be accomplished (as illustrated inFIG. 2, where B has a higher magnification than A) in close proximity toa pre-primary assembly (such as a nanofiber forest). In such cases arectangular nanotube ribbon does not form prior to the twist process.Alternatively, a rectangular nanotube ribbon can be drawn from theforest and twisted into a yarn after the rectangular ribbon issubstantially formed.

It is important to note in FIG. 2 that a spinning triangle is formed asthe nanotubes are drawn from a forest and progressively converge to forma yarn having a substantially circular cross section. The geometry ofthe spinning triangle down to the nanoscale is related to the width ofthe nanofiber forest sidewall (i.e., edge) selected for spinning, theforest height, and the helix angle needed for the spun yarn (which is inturn determined by the ratio of yarn twist rate to yarn draw rate).Since the strength of the nanofiber array in the convergence zone (i.e.,the spinning triangle) is lower than that of the yarn, it is optionallypreferable that convergence to a partially twisted yarn havingapproximately circular cross-section be substantially complete withinabout 50 millimeters of the nanofiber forest or a pre-primary assemblyof any sort. Optionally and more preferably, this substantially completeconvergence to produce the tip of the spinning triangle occurs withinabout 5 mm from the nanotube forest. The optimal convergence distancealso depends on forest height. Optionally and preferably, thissubstantially complete convergence to partially twisted yarn havingapproximately circular cross-section occurs within a distance from ananofiber forest that is less than 50 times the average height of thenanofiber forest. More optionally preferable, this substantiallycomplete convergence occurs within a distance that is less than about 5times the average height of the nanofiber forest or a pre-primaryassembly of any sort.

It is also important to note in FIG. 2 that initial twisted coreformation is evident at approximately the center of the wedge that isbeing twisted during draw. It is optionally preferable that yarn coreformation appears ¼ to ¾ of the distance between wedge end to wedgeapex. It is also optionally preferable that the twisted fiber coreappears at between ¼ and ¾ of the lateral distance between the lateralwings of the wedge.

The Inventors find, surprisingly, that the draw angle (between the drawdirection and the direction of the nanofibers in the forest) canusefully vary between 90° and nearly 0° (nearly orthogonal to the planeof the substrate and attached forest). For some spinning processes, thedraw angle is preferably between about 90° and 60°, and for otherprocesses the draw angle is preferably between about 0° and 50°.

The nanofiber forests can optionally be stripped from the growthsubstrate and spun into nanofiber yarns, ribbons, or sheets while notattached to this growth substrate. This stripping process can optionallyoccur during the spinning process. Where a forest substrate does notrestrict the draw angle, the draw angle can optionally and preferablyvary over the range from 85° above the plane of the forest to 85° belowthe plane of the forest.

Nanofiber forests stripped from the growth substrate can optionally bestacked upon each other to provide an array of layers from which thenanofiber yarns are spun (see Example 43). These nanofiber layers can beoptionally mechanically compressed orthogonal to the plane of theforests to provide a degree of interpenetration between nanofibers inneighboring nanotube forest layers. The forest layers in the laminatednanotube forests can optionally comprise nanotubes having the sameheight and density within the forest, or they can differ in nanotubeheight and density within the forest, the chemical composition orstructure of the nanotubes, or optional coating materials or frictionaids within the individual nanotube forests.

The nanofibers in neighboring contacting forests can optionally beterminated with reactive groups that cause end-to-end or near-endsidewall binding between individual nanofibers or nanofiber bundles indifferent layers. Such binding processes can enable faster spinningrates than are otherwise possible and improve the properties of the yarnby effectively increasing the nanofiber or nanofiber bundle lengths.

The draw-twist spinning process can be conveniently and optionally andpreferably practiced at close to ambient temperature, i.e., ordinaryroom temperature or temperatures reached without intentional heating orcooling. However, in some embodiments it is optionally preferred thatthe draw-twist spinning process is accomplished at either above or belowambient temperature. For example, such higher or lower temperatures canoptionally be employed to optimize the degree of direct or indirectinter-fiber bonding for the draw-twist process. For example, aninter-fiber binding aid can optionally be employed to provide aself-supporting capability for nanotube forests that have been strippedfrom the growth substrate. Local heating at close to the edge of thenanofiber forest can be usefully employed for evaporating the bindingaid or causing this binding aid to become fluid, so that the draw twistprocess can occur most productively. Also, said heating process can beused for reaction with gas phase additives that facilitate thedraw-twist process or improve yarn properties. This heating can beaccomplished by various means, such as using heating induced by visible,ultraviolet, infrared, radio frequency, or microwave frequencyabsorption or resistive heating using solid- or gas-phase contactheating (and combinations thereof). The heating or cooling processesoptionally provide a nanofiber temperature of between −200° C. and 2000°C. Optionally and more preferably, this heating or cooling process isbetween −20° C. and 500° C. Optionally and most preferably for someinvention embodiments, the initial draw step and the initial twist stepto form the twisted nanofiber yarn is conducted at below 60° C. Localheating for regions being spun, such as the edge of a nanotube forest,can usefully be accomplished via resistive heating by using a voltageapplied between the spun yarn and the nanotube source, which results ina current along the nanofiber yarn.

One particularly advantageous arrangement is to synthesize thenanofibers as a forest on a surface that continuously moves from afurnace region (where CVD is used to grow the nanofibers as a forest)into a region where the nanofibers in the forest are either draw-twistor nanofiber ribbons or sheets are drawn from the nanofiber forest. Themethod of growth of the nanofiber forest in the furnace region can be bythe various methods known in the art, and variations on these methods,such as shown in Example 1.

In one optional and preferred method the growth substrate is either aflexible belt or is attached to moving belt. This moving belt carriesthe nanofiber forest from the forest growth region of the productionapparatus to the region where yarns are twist spun or nanofiber ribbonsor sheets are spun. The spun twisted nanofiber yarns, nanofiber ribbons,or nanotube sheets can be optionally transferred in a continuous processto manufacture steps where the nanofibers yarns ribbons are optionallyplied, the nanofiber ribbons or sheets are laminated, and where thenanofiber yarns, ribbons, or sheets are optionally overcoated orinfiltrated with an agent (such as a polymer) that serves any of variousfunctions (such as to increase the bonding between nanofibers, toprovide electrical insulation, or to provide ionic conductivity for useof these yarns, ribbons, and sheets for electrochemical deviceapplications).

Materials that are both suitable as belt or drum materials and suitablefor nanotube forest growth are known in the literature. For instance,Ch. Emmenegger et al. report (Applied Surface Science 162-163, 452(2000)) that aluminum and cobalt are suitable substrates for nanotubegrowth. In addition, L. Liang et al. (U.S. Patent ApplicationPublication No. 20040184981 A1) describe the application of the oxidizedsurfaces or many metals for the growth of nanotube forests, where thebenefit of the metal oxide layer is to prevent the deactivation of thecatalyst used for forest growth.

Another suitable drum material is an outer layer of amorphous SiO₂ orcrystalline SiO₂ (quartz), which is known to be an effective substratematerial for nanotube forest growth, as well as the growth of al leasteight forests upon each other (X. Li et al., Nano Letters 5, 1997 (2005)and Y. Murakami, et al. Science 385, 298 (2004)). The SiO₂ is alsosuited for application on belts used for nanotube forest growth (andstripping to form yarns, sheets, and ribbons), as long as the SiO₂ layeron the belt is sufficiently thin, relative to the maximum radius of beltcurvature (usually corresponding approximately to the radius of rollersused to move the belt) such that fracture of the SiO₂ does not occur.

In some embodiments, nanotube synthesis to make a primary or pre-primaryarray, and the nanotube spinning can be accomplished as a continuousprocess by employing a rotating drum. This drum is preferably at least50 centimeters in diameter. Synthesis of the nanofibers (such as forestsmade by CVD processes) on one side of the drum is followed by nanofiberyarn draw and subsequent twist-based, false-twist-based, orliquid-densification-based spinning processes to make yarn or thedrawing of ribbons or sheets from a distant part of the rotating drum.

As another alternative, the fabrication of the primary or pre-primarynanotube array and the nanotube spinning can be accomplished indifferent apparatus, such as by coiling a substrate containing theprimary or pre-primary nanofiber array into a roll. This roll can thenbe unwound as a separate process for conducting spinning, and especiallydraw-twist spinning processes.

Example 37 demonstrates such a process wherein forest-spun nanotubesheets are attached to a plastic film substrate, densified on thissubstrate using liquid infiltration and evaporation, and then draw-twistspun into a carbon nanotube yarn by drawing a nanotube ribbon from theplastic film substrate and twisting this ribbon.

The nanofibers on the primary assembly or the pre-primary assembly canbe optionally patterned, and patterned depositions of differentnanofibers on the same substrate can be optionally employed and thesedifferent nanofibers can be optionally spun into either the same yarn ordifferent yarns. Such patterned deposition of nanofibers, which can beobtained, for example, by the patterned deposition of nanofiber growthcatalyst, can be used to help determine the diameter of twisted yarn orthe width of spun ribbons. For the mentioned invention embodiments inwhich moving belts or rotating drums are used, this patterned depositionof nanofibers is preferably as parallel strips that extend in thedirection of substrate displacement.

As a useful alternative to patterned deposition of nanotubes, thenanotubes can be grown uniformly over the substrate (such as by CVDdeposition of a nanotube forest on the surface of a drum or belt) andlaser trimming can be used for patterning the nanotube array forsubsequent draw-twist spinning. This laser trimming is optionally andpreferably such that narrow lines of removed nanotubes separate parallelregions where the nanofiber forest is largely unexposed to the laserbeam. These lines of removed nanotubes are preferably parallel to thetranslation direction resulting from drum rotation or belt translation.Laser trimming can also be used to uniformly decrease the height ofnanofibers in a forest of nanofibers. The benefit of such trimming ofnanofiber height along the length of a strip of nanofiber forest usedfor spinning is to control yarn structure along the length of draw-twistyarns. Such trimming of nanotube height along a forest strip length canbe optionally done periodically, so that the produced draw-twist fibershave periodic variation in structure along the fiber length. Suchvariation in yarn structure along the yarn length can be useful forachieving property variations along this length, such as electronicproperty variations or for increasing the yarn density by providing thinyarn segments that can fit in void spaces between segments of largerdiameter yarns. The yarn structures having variations along the yarnlength are preferably either twisted yarns, false-twisted yarns, liquiddensified yarns, or non-twisted yarns that are infiltrated with an agentthat provides inter-nanofiber binding, such as an organic polymer.

As will be described below in more detail, the effect of the abovechange in nanofiber length for different segments of the yarn is tochange the local density of the twisted yarn, and this change in localdensity can be employed for directing processes that selectivelytransform the electrical properties of one yarn segment relative to thatof other yarn segments. The effect is that various electronic devicescan be made along the yarn length, such as diodes based on n-pjunctions. Various processes can be used for this selective areapatterning, which make use of the effect of local density changes. Thelocal density of a particular yarn segment depends upon the length ofthe nanofibers in that segment, since the nanotube length affectsinter-fiber bonding, and thereby the distribution of twist betweendifferent segments of the yarn—which affects the yarn density (porosity)for a segment and the dependence of local yarn density on an appliedtensile strain. The electrical conductivity of a segment, and thetemperature rise caused by a current through the yarn, is also affectedby the porosity differences between segments and the varying number ofinter-fiber contacts per nanotube.

Porosity differences along the yarn can be used for selectively dopingdifferent yarn segments, selectively chemically modifying them, or forprotecting one yarn segment from chemical exposure effects that affectother yarn segments (by selective infiltration of protecting chemicalsin different yarn segments). These processes form the initial basis foryarn lithography processes of device embodiments, which enable theconstruction of electronic devices in the nanofiber yarns (see Section10(a)). The porosity differences between different yarn segments havingdifferent structure can be tuned by varying the tensile stress appliedto the yarn. Also useful for this new type of yarn lithography, theselective heating of different yarn segments, due to the porositydifferences and resulting electrical conductivity differences of thesesegments, can be used for the selective deposition, selective reaction,or selective removal of particular chemicals from specific yarnsegments.

Various agents can be used to modify the properties of, and interactionsbetween, nanotubes during processing; the final yarn, ribbon, or sheet;or the properties of the intermediate or final products made of orincorporating said yarn, ribbon, or sheet. Such agents can be selectedto optimize yarn properties including, but not limited to, friction orbinding, strength, thermal and electrical conductivity, chemicalreactivity, and surface energy and chemistry.

Suitable agents can provide the desired function when in either thesolid state, liquid state or adsorbed gas state and can be appliedeither from the gas, vapor or liquid state, from a gas plasma, from asuspension, solution, dispersion, emulsion or colloid, electrochemicallyfrom a solution, or by particle, fiber or layer infiltration and byother methods familiar to those skilled in the art of application. Theseagents can be applied to a pre-primary assembly like a nanotube forest,to the primary assembly, or after formation of the twisted yarn, ribbon,or sheet. Agents for the chemical or physical modification of carbonnanotubes and inter-nanotube interaction in nanotube forests for yarn,sheet, or ribbon fabrication processes are optionally and preferablydelivered from gas phase, vapor phase, or plasma states.

Agents used for modifying the properties of the pre-primary assembly,primary assembly or yarn, ribbon, or sheet can be selected to physicallyor chemically modify the surface of the nanotube fibers, as inoxidation, reduction, or substitution with functional groups, such as by(1) covalently binding molecular, polymeric, or ionic species to thenanotubes; (2) forming non-covalent binding, as in van der Waals andcharge-transfer binding (3) covalently or non-covalently binding speciescapable of hydrogen bonding, and/or (4) physically over coating with apolymer, a metal or metal alloy, a ceramic, or other material. Agentscan be selected that, irrespective of bonding, at least partiallyencapsulate, envelop, or coat individual nanotubes or bundled nanotubeson the nanoscale.

Irrespective of the nature of any bonding, agents can be selected so asto have one or more physical dimensions of a similar order to thenanotubes, that is to say nanolayers, nanofibers and nanoparticles, andthereby or otherwise to interpose between the nanotubes and produce avariety of physical or chemical interactions between them. Suchinteractions can encompass, but not be limited to, locking tubestogether, or facilitating their relative motion, or of facilitating orlimiting the transfer of electrical or thermal or light or sound energybetween them or of facilitating or limiting the transfer of strain orcompression or shear or rotary force between them. Irrespective of theirsize and the nature of their interactions, agents can be selected whichinterpose to separate nanotubes and thereby limit or facilitate theirinteractions, or which occupy the interstices between nanotubes but donot interpose or separate them and hence or otherwise allow orfacilitate direct inter-tube contact.

Notwithstanding that agents mentioned heretofore are applicable tointeractions between individual or bundled nanotubes within apre-primary assembly, primary assembly or yarn, ribbon or sheet, allsuch agents are also similarly able to facilitate or limit interactionbetween said pre-primary assembly, primary assembly or yarn, ribbon orsheet and the externality including, but not limited to, the substrateon which the nanotubes are grown, the tools and equipment used toproduce, handle, process or store them and the intermediate or finalproducts into which they are fashioned or in which they areincorporated, including but not limited to yarns, textiles andcomposites. Such interaction with the externality can include deliberateconnection to the externality by, but not limited to, the methods ofbonding, soldering, welding, attaching, connecting and other suchmethods used by those skilled in the art of connection. Said interactioncan also include deliberate prevention of connection in the nature of,but not limited to, insulating, isolating, masking, desensitizing orrendering incompatible.

Agents can be selected which are applied and present only for thatoperation for which they are intended and then are removed or otherwisedepart having fulfilled their function. Such agents may be washed outwith a solvent, liquefied, evaporated or decomposed by thermal energy,decomposed or altered by any form of radiation or chemical treatment, orotherwise be rendered, soluble, mobile, labile, volatile or fugitive inorder that such agent may wholly or substantially be removed from orleave the nanotubes. Agents can be selected which are applied andpresent for a particular function but which then remain either servingno further purpose, or continuing to serve their initial function, orserve alternative or additional functions in subsequent operations andintermediate and final products. Such agents may remain wholly unchangedor may undergo chemical or physical changes or both. An example of suchan agent is a chemical which, in its monomeric form acts as a lubricantor friction modifier for yarn assembly and is subsequently polymerizedin situ to promote or facilitate nanotube or yarn adhesion andinteraction.

Those skilled in the art will recognize that the agents described hereinfulfill many of the functions applied in conventional fiber processingand are of recognized types including, but not limited to, fillers,surfactants, lubricants, modifiers, humectants, binders, sizes, linkers,adhesives, monomers and polymers. Those skilled in the art will alsorecognize that the applications of these agents to a pre-primaryassembly, primary assembly or yarn, ribbon or sheet of nanotubes or tointermediate or final products made of or incorporating them introduceunique and hitherto undiscovered or unobtainable qualities and functionsto them.

The pre-primary and primary nanofiber assemblies and the final twistedyarns can optionally include (1) nanofibers having substantiallydifferent lengths or diameters, (2) non-nanosize diameter fibers thatare either continuous or limited in length, (3) nanofibers havingdifferent chemical or physical surface treatments, or (4) nanofibersthat have effectively continuous lengths. One benefit of includingcontinuous or effectively continuous twisted fibers in the twisted yarnsis that these effectively continuous fibers can help bind short lengthnanofibers into a mechanically robust assembly. Optionally and mostpreferably, these effectively continuous fibers are also eithermicrodenier fibers (weighing less than a gram per 9000 meter length) ornanofibers. These nanofibers having effectively infinite length arepreferably made by electrostatic spinning. These continuous oreffectively continuous fibers optionally and preferably largely compriseeither a metal or an organic polymer.

One preferred method to spin a singles yarn that comprises nanofibershaving different lengths, different chemical compositions, or differentcoatings is to effectively and simultaneously draw-twist these fibersfrom the same pre-primary or primary assembly. This pre-primary orprimary assembly is optionally and preferably a nanofiber forest.

It is optionally preferred for selected applications that the twistednanofiber yarns comprising different fiber components are assembled in asegregated manner, such as alternating strips in a nanofiber forest.

(b) False-Twist-Based Yarn Spinning from Nanofiber Forests

The Inventors have surprisingly discovered that a substantial part ofthe mechanical strength enhancement due to twist-based spinning ofnanofibers can be obtained by using false twist. False twist isbasically twist in one direction followed by an approximately equaltwist in the opposite direction. This unexpected discovery has enormouspractical importance for several reasons. First, false twist can beintroduced very rapidly, which decreases the cost of spinning processes.Second, false-twisted nanofiber yarns can be advantageously used forformation of yarns in which twist is not needed to provideinter-nanofiber coupling, which can be the case for yarns in whichnanofiber length is very long and ones in which an infiltrated material(such as a polymer) provides mechanical coupling between nanofibers. Forexample, the discovered strength enhancement due to false twist enablesapplication of the higher stresses needed for fast spinning, whether ornot true twist (net twist in one direction) is later introduced into theyarn. Finally, the absence of significant true twist for ananotube/polymer composite yarn can enhance yarn toughness (the energyrequired to break the yarn), since the presence of substantial twist caninterfere with the energy dissipative processes that otherwise occurover large yarn deformations.

The experiments of Example 40 show that twist dramatically increasesyarn tensile strength, even when this twist is subsequently eliminatedby an equal twist in an opposite direction. In this experiment, ribbonshaving fixed width were pulled from a carbon nanotube forest. In theabsence of twist or false twist, the strength of the ribbon was too lowto be measured. When twisted to form a twist angle of 280, strengthincreased from this negligible value to 339 MPa. However, unlike thecase for yarns comprising large diameter fibers, an important percentageof this strength increase (33%) was retained when the carbon nanotubeyarn was subsequently untwisted by an amount equal to the initial twist.Note that the increase in yarn diameter (compare A and B SEM micrographsof FIG. 52) as a result of twist de-insertion is relatively small.

Since strong untwisted yarns are highly desirable for use in formingnanotube/polymer composite yarns have both high strength and hightoughness, this surprising discovery that false twist (twist insertionfollowed by twist de-insertion) can dramatically increase yarn strengthis quite important. This discovery provides the motivation for thefalse-twist spinning apparatus described in FIGS. 44-46.

False twist processes can optionally be applied more than once to ayarn, so as to provide yarn densification and other desirable results.Also, twist-based spinning and liquid-densification-based spinning (seeSection 1(e)) can be optionally and beneficially applied duringnanofiber yarn spinning.

(c) Sheet and Ribbon Fabrication from Nanofiber Forests

While drawing yarns from carbon nanotube forests have been described inthe prior art, these yarns have a maximum reported width of only 200 μmand are much too weak to be useful. The Inventors herein show thatstrong sheets having arbitrarily wide widths can be drawn from nanotubeforests.

The structural nature of the nanotube forest is important for drawingboth sheets and wide ribbons from nanotube forests, and the preferredstructural nature of the forest is described in Section 1(e).

Example 21 demonstrates the drawing of about five centimeter widthtransparent nanotube sheets from the sidewall of multiwalled nanotube(MWNT) forest. Draw was initiated using an adhesive strip to contactMWNTs teased from the forest sidewall. Importantly, bundled nanotubeswere simultaneously pulled from different elevations in the forestsidewall, so that they join with bundled nanotubes that have reached thetop and bottom of the forest, thereby minimizing breaks in the resultingfibrils (FIGS. 22 and 23). Sheet production rates of up to ten metersper minute where demonstrated, which is comparable to the rates usedcommercially for twisting wool together to make yarn. Even when themeasured areal density of the sheet was only ˜2.7 μg/cm², meter-long,500 cm² sheets were self-supporting during draw. A one centimeter lengthof 245 μm high forest converted to about a three-meter-longfree-standing MWNT sheet. The sheet fabrication process is quite robustand no fundamental limitations on sheet width and length are apparent:the obtained 5 cm sheet width equaled the forest width when the drawrate was about 5 m/min or lower. The nanotubes are highly aligned in thedraw direction, as indicated by the striations in the SEM micrograph ofFIG. 22.

For applications in which sheet or ribbon transparency is needed, thecarbon nanofiber sheets or ribbons preferably have an areal density ofless than 10 μg/cm².

As for twist-based nanofiber draw, it is preferred that nanofibers aresimultaneously pulled from essentially the full height of the nanofiberforest sidewall (edge).

For economic reasons, the ribbon and sheet draw processes are optionallypreferably conducted at least 5 meters per minute. Also, for reasons ofeconomic fabrication, the nanotube sheets optionally have a width ofabout 5 cm or greater.

Example 22 shows that the solid-state drawn nanotube sheet of Example 21comprises a novel, useful state of matter that was previously unknown:an aerogel comprising highly oriented carbon nanotubes. From themeasured areal density of about 2.7 μg/cm² and the sheet thickness ofabout 18 μm, the volumetric density is approximately 0.0015 g/cm³.Hence, the as-produced sheets are an electronically conducting, highlyanisotropic aerogel that is transparent and strong.

The high degree of nanotube orientation in the nanotube sheet isdemonstrated by the Raman spectra of FIG. 41, which indicates apolarization degree of about 0.69 to 0.75. The anisotropy of lightabsorption (FIG. 25) also indicates the high anisotropy of the nanotubesheets. Ignoring the effect of light scattering, the ratio of absorptioncoefficient for parallel and perpendicular polarizations for theas-drawn single sheet was 4.1 at 633 nm, and monotonically increased to6.1 at 2.2 μm. The striations parallel to the draw direction in the SEMmicrograph of FIG. 22 provides more evidence for the high degree ofnanotube orientation for the as-drawn nanotube sheets.

For certain applications, it is optionally preferred that the aerogelsheets and ribbons made by invention embodiments have a density of lessthan 0.005 g/cm³.

The width of the nanofiber sheet can be optionally increased ordecreased to ribbon type widths. This can be optionally accomplished bycontrolling the width of the nanotube forest sidewall (or otherpre-primary nanofiber assembly) that is contacted when ribbon draw isinitiated, patterning forest deposition, or by separating wide drawnsheets into ribbons (such as by mechanical or laser-assisted cutting).The ribbon width is optionally preferred to be at least 0.5 mm. Moreoptionally preferred, the ribbon width is above one millimeter.

In another method of invention embodiments, nanofiber sheets ofarbitrarily large lateral extent are obtained by assembling nanofiberribbons or narrower sheets, so that adjacent ribbons or narrower sheetsat least partially overlap to provide inter-ribbon binding. Thisassembly can be accomplished on a planar or non-planar substrate, suchas a rotating drum. Since the inter-ribbon binding will normally be low,a binding agent (such as a polymer like polyvinyl alcohol) canoptionally be used to enhance inter-ribbon bonding in the sheet.Alternatively, inter-ribbon bonding can be enhanced by other means, suchas by using electron beam, microwave, or radio frequency welding(optionally in the presence of a bonding agent). Soaking the sheet inliquid, such as methanol or isopropyl alcohol, and then drying (seesection 1(d)) is another way to fix the binding (inter-ribbon bindingand/or the binding between ribbon and substrate).

(d) Liquid-Based Densification for Strengthening Nanotube Sheets andRibbons

The Inventors have also surprisingly discovered that liquid imbibing,followed by liquid evaporation can be used for causing over 300 folddensification of nanofiber sheets an ribbons, and for increasing bothstrength and tenacity (gravimetric strength).

More specifically, Example 23 shows that the Inventors can easilydensify these highly anisotropic aerogel sheets into highly orientedsheets having a thickness of 50 nm or less and a density of ˜0.5 g/cm³.In this specific instance they obtain a 360-fold density increase bysimply adhering by contact the as-produced sheet to a planar substrate(e.g. glass, many plastics, silicon, gold, copper, aluminum, and steel),immersing the substrate with attached MWNT sheet into a liquid (e.g.ethanol), retracting the substrate from the liquid, and then permittingevaporation. Densification of the entire sheet, or selected areas withinthe sheet, can also be similarly obtained by dropping or otherwiseinjecting such a liquid onto the sheet area where densification isdesired, and allowing evaporation. Surface tension effects duringethanol evaporation shrank the aerogel sheet thickness to 30-50 nm forthe MWNT sheet prepared as described in Example 1. The aerogel sheetscan be effectively glued to a substrate by contacting selected regionswith ethanol, and allowing evaporation to densify the aerogel sheet.Adhesion increases because the collapse of aerogel thickness increasescontact area between the nanotubes and the substrate.

Example 27 shows that the densification process substantially increasesthe mechanical properties of the nanotube sheets. Stacks of undensifiedidentically oriented sheets have an observed gravimetric tensilestrength of between 120 and 144 MPa/(g/cm³). A densified stackcontaining identically oriented sheets had a strength of 465MPa/(g/cm³), which decreased to 175 MPa/(g/cm³) when neighboring sheetsin the stack were orthogonally oriented to make a densified biaxialstructure. These density-normalized strengths are comparable to orhigher than the ˜160 MPa/(g/cm³) strength of the Mylar® and Kapton®films used for ultra-light air vehicles and proposed for solar sails forspace applications (see D. E. Edwards et al., High Performance Polymers16, 277 (2004)) and those for ultra-high strength steel sheet (˜125MPa/(g/cm³)) and aluminum alloys (˜250 MPa/(g/cm³)).

Example 35 shows that the nanotube sheets are a type of self-assembledtextile in which nanofiber bundles branch and then recombine with otherbranches to form a network having a degree of lateral connectivityorthogonal to the draw direction. The SEM micrograph of FIG. 28 showsthis branching and branch recombination. Fibril branching continuesthroughout the sheet, thereby making a laterally-extended, inherentlyinterconnected fibril network.

(e) Liquid-Densification-Based Spinning from Nanofiber Forests

The inventors find that a strong nanotube yarn can be obtained from ananotube forest by using liquid-based densification, thereby avoidingthe need for either twist or false twist. While Example 38 demonstratesthis process for a ribbon drawn from a forest, the inventors find thatthis process can also be applied for narrow yarns. If no twist isapplied and the yarns are used as-drawn from the forest, the yarnmechanical strength was too low to be measured using the availableapparatus. The effect of liquid treatment (involving liquid imbibing,and filament densification during liquid evaporation) was todramatically increase strength, as well as to increase tenacity (seeExample 20). For the case of the ribbon described in Example 36, theobtained strength resulting from liquid-densification was 215 MPa.

The choice of liquids for liquid-densification-based nanotube spinningis guided in part by surface energy and liquid cohesive energyconsiderations, since it is desirable that the imbibed liquid isadequately imbibed into the nanotube sheet, ribbon, or yarn. Since thesurface energy of the nanotubes can be dramatically effected by chemicalderivatization and surface coatings (see Section 7), this choice ofliquid depends upon whether or not this derivatization has occurred (forexample, as a result of reactions during forest synthesis,post-synthesis treatment of the nanotube forest, or after initiallydrawing the nanotube ribbon, sheet, or yarn). For the largelynon-derivatized nanotube forests of Example 1, acetone, ethanol,methanol, isopropyl alcohol, toluene, chloroform, and chlorobenzenepreform well as liquids for sheet, ribbon, or yarn densification. Theperformance of liquids that do not perform well for a particular type ofnanotube can be enhanced by adding a suitable surfactant. For example,water does not perform satisfactorily for densifying the nanotube sheetsprepared using the method of Example 22 from the nanotube forests ofExample 1. However, a surfactant/water mixture (either 0.7 weightpercent Triton X-100 in water or 1.2 weight percent lithium dodecylsulfonated in water) was a satisfactory densification agent (see Example23). Other considerations for the choice of liquid for densification areliquid viscosity (which affects the rate of the liquid infiltrationprocess) and the ease at which this liquid can be volatilized duringsubsequent processing.

The optionally preferred degree of infiltration is optionally andpreferably the maximum that can be achieved without unnecessarilyincreasing processing costs. However, it is sometimes useful to obtainyarns, sheets, and ribbons in which liquid is imbibed only into theouter surface regions of these articles. The benefit of such partialinfiltration is to obtain densification for principally the imbibedregions.

Various methods can be usefully employed for achieving infiltration ofthe liquid used for densification into the nanofiber yarn, ribbon, orsheet. These include among other possibilities the condensation of avapor, immersion into the liquid, and exposure to an aerosol of theliquid. Removal of the densification liquid is preferably byevaporation. Supercritical fluids can also optionally be used as liquidsfor achieving yarn, sheet, or ribbon densification.

The liquid used for densification can optionally contain a binding agentor other functionally useful agent for enhancing yarn properties (seeSection 8), which can be either dissolved in the densification agent ordispersed within it as colloidal material. Useful types of colloidalparticle include catalyst particles and nanofibers, especially largelyunbundled carbon single walled nanotubes.

Twist-based spinning, liquid-densification-based spinning (see Section1(e)), and false-twist based spinning can be optionally and beneficiallyapplied in any combination during yarn spinning. Also,liquid-densification-based spinning can be optionally combinedsimultaneously with twist-based spinning—and the twist can either beretained or subsequently be partially or completely removed during lateryarn processing. The inventors show in Example 38 that densification ofdrawn ribbons prior to twist made it possible to obtain uniformlytwisted singles yarn even when the applied twist is very low(corresponding to a helix angle of 50). Application of such low twist inthe absence of pre-applied liquid-based yarn densification resulted innon-uniform twist and yarn diameter.

(f) Elaboration of Nanofiber Forest Types Useful for Yarn, Sheet, andRibbon Production

Most types of nanotube forests are either unsuitable for spinning orproduce weak yarns or ribbons. The nanotube forests used for spinningeither have a degree of entanglement or other bonding between thesubstantially parallel nanotubes in the forest, or one that developsearly in the spinning process. A degree of bundling in the forest,wherein one nanotube meanders between different bundles, can be obtainedby using such type of CVD forest growth processes as described inExample 1. Such a degree of bundling and meandering is preferable. It ismore specifically preferable that nanotubes in the forest undergointermittent bundling, meaning that a individual nanotube forms smallbundles with a small group of neighboring nanotubes at one locationalong the forest height and with other small group of neighboringnanotubes at other locations along the forest height.

Transition from spinable to non-spinable or difficultly spinable forestscan be caused by relatively minor changes in the reaction conditionsused for nanotube forest growth. Even changing the furnace size and typeused for spinning can importantly change the spinability of forests andthe ease of drawing forests. However, those having ordinary skills inthe known art of forest growth will be able make small changes in growthconditions, so as to provide useful nanotube forests for nanotube sheet,ribbon, and yarn production.

Too low a density of nanotubes in a forest renders a forest difficult tospin. This is illustrated in FIG. 56 where SEM micrographs of the growthsubstrates are compared for spinable and practically non-spinablenanotube forests (after removal of the nanotubes) wherein the smalldiameter pits on the growth substrate correspond to the growth site ofMWNTs. The nanotube diameters (about 10 nm) are roughly the same forboth of these spinable and practically non-spinable forests. However,the inventors have observed nanotube forest base area densities of 90billion to 200 billion nanotubes/cm² for nanotube forests that arehighly spinable, as compared with 9 billion to 12 billion nanotubes/cm²for low density nanotube forests that were difficult or impossible tospin. Also, the inventors observed that the percentage of the forestbase area that was occupied by nanotubes was much higher (7% to 15%) forhighly spinable forests, as compared with 1.1% to 2.5% for nanotubeforests that were difficult or impossible to spin.

While these measurements of nanotube density and fraction of forest areathat is occupied are most conveniently measured using the base plane, itshould be understood that nanotube forest density can differ from thebase plane value, both increasing because individual nanotubesprematurely stop growth and because new nanotube growth is initiatedabove the forest base. Optionally and preferably at least 20% of thenanotubes initiated on the base area continue growth to essentially thetop of the forest. Optionally and more preferably, at least about 50% ofthe nanotubes initiated on the base area continue growth to essentiallythe top of the forest.

Reflecting these complications the terms maximum nanotube forest densityand maximum % of forest area are used, which are defined as the maximumvalues of these parameters when measured on planes parallel to thegrowth surface. Also, in some cases it is useful to employ a non-planargrowth surface. In which case, the terms nanotube forest density andfraction of surface occupied by nanotubes is defined using either thenon-planar growth surface or for a surface essentially parallel to thegrowth surface.

Based on these surprising observations, nanotube forests used directlyfor yarn, ribbon, or sheet draw preferably have a maximum nanofiberdensity of at least 20 billion nanotubes/cm² when the nanotube diameteris approximately 10 nm. More generally for these and other nanotubediameters, nanotube forests used directly for yarn, ribbon, and sheetdraw have a maximum percentage of the forest area that is occupied bynanotubes that is optionally and preferably above about 4%. Optionallyand more preferably, the nanotube forests used directly for yarn,ribbon, and sheet draw have a nanofiber density on the forest base of atleast 20 billion nanotubes/cm² when the nanotube diameter isapproximately 10 nm. Also, optionally and more preferably, the nanotubeforests used directly for yarn, ribbon, and sheet draw have a percentageof the forest base area that is occupied by nanotubes that is aboveabout 4%.

Too high a density of nanotubes in the forest and too large aninteraction between the nanotubes in a forest can also make nanotubeforests difficult or impossible to spin. The problem here is thatinteractions within the forest are so strong that the draw-inducedtransformation from nanotube orientation in the forest to that in theyarns and sheets is interrupted, and principally clumps of nanotubes arewithdrawn from the forest.

Nanotube forests used directly for yarn, ribbon, and sheet draw have amaximum percentage of the forest area that is occupied by nanotubes thatis optionally and preferably less than 40%. Also, optionally and morepreferably, the nanotube forests used directly for yarn, ribbon, andsheet draw have a percentage of the forest base area that is occupied bynanotubes that is below about 40%.

The product of the number of nanotubes per unit area in the forest andthe nanotube diameter is optionally and preferably in the range between0.16 and 1.6 when measured on the forest base, since this parameterrange is especially useful for sheet, ribbon, and yarn spinning fromcarbon nanotube forests.

This nanofiber forest suitable for formation of the primary assembly, oras a primary assembly, can be on a planar or non-planar substrate andthe nanofibers in the forest can either be deposited over substantiallythe entire substrate surface or only on part of the surface. Also,different types of nanotubes can be in different regions of the surfaceor can be mixed in the same forest regions.

For convenience in facilitating nanotube yarn spinning and ribbon andsheet draw, the minimum radius for the forest-occupied-area of a curvedforest substrate is optionally over ten times the maximum height of theforest. Use of a curved surface substrate, whether or not this substrateis subsequently removed before ribbon or sheet draw, can facilitate thedraw of ribbons and sheets that are non-planar. For the purpose ofdrawing such ribbons and sheets, the tool used to start spinning or drawshould preferably have a matching shape, meaning that this tool issufficiently matched in shape to the curved substrate that appropriateforest contact can be made every where to start spinning.

(g) Elaboration on Methods for Starting Sheet, Ribbon, and Yarn Draw

Example 46 describes methods for initiating the drawing of a nanotubesheet, ribbon, ribbon array, yarn, or yarn array from a nanotube forestusing an adhesive, an array of pins, or a combination of an adhesive andan array of pins. Interestingly, the inventors find that contact of anadhesive tape to either the top or sidewall of the nanotube forest isuseful for providing the mechanical contact that enables the start ofsheet draw. Any of an enormous variety of adhesive tapes and adhesivesapplied to surfaces are suitable, including the various Scotch® brandadhesive tapes of 3M and the adhesive strip of 3M Post-it® Notes.Contact of a straight adhesive strip (so that the adhesive strip isorthogonal to the draw direction) is especially efficient for startingthe draw of a high structural perfection sheet. The reason that this topcontact method is especially advantageous is that nanotube foreststypically have non-straight sidewalls, and the use of a straightadhesive strip (or a straight array of suitably spaced pins) providesstraight contact for forest draw to make a sheet.

An array of closely spaced pins can also be usefully employed to startsheet draw. In one experiment, the pin array consisted of a single lineof equally spaced pins. The mechanical contact needed for spinning wasin this case initiated by partial insertion of the linear pin array intothe nanotube forest (see Example 46). The pin diameter was 100 micron,the pin tip was less than one micron, and the spacing between the edgesof adjacent pins was less than a millimeter. Satisfactory sheet draw wasachieved using pin penetration of between ⅓ and ¾ of the height of theforest (in the range between 200 and 300 microns).

Optionally and preferably, neighboring pins in the pin array can havediffering lengths, so that they are inserted to differing depths in thenanotube forest. Also, instead of using a single linear array of pins,the pin array can be two dimensional in lateral extent. For example thepin array can beneficially consist of two or three rows of pinsperpendicular to the draw direction in which adjacent rows areoptionally offset in the row direction by one-half of the inter-pinspacing in the row direction. The pins in the pin array are optionallyand preferably approximately equidistant from nearest neighbor pins.

Draw processes of multiple ribbons or yarns can be similarly initiatedusing a linear array of adhesive patches or a linear array of pins thatare separated into segments. The separation distance between adhesivepatches along the length of the linear array determines the ribbon oryarn width. The yarn can be subsequently strengthened by, for instance,twist-based spinning, false-twist spinning, liquid-densification-basedspinning, or any combinations thereof. Mechanical separation of thesheet strip patches or the pin patches in the linear array during thestart of draw is usefully employed in order to avoid interference duringprocessing of adjacent ribbons or yarns, such as during the introductionof twist.

Use of adhesive patches (or pin patches) that have different lengthsalong the strip direction can be usefully employed—such as to draw-twistadjacent strips to produce different diameter yarns (which can beoptionally combined to provide a plied yarn in which different singlesyarns in the plied yarn have different diameters).

Different degrees of twist or directions of twist can be convenientlyand usefully applied to different singles yarns that are drawn using thesegmented adhesive or pin strip, and these different singles yarns canthen be optionally plied together in a yarn containing a freely-selectednumber of plies. Importantly, using the above methods for introducingdifferent diameter singles yarns into a plied yarn can be employed toproduce a plied yarn having enhanced density, since smaller diametersingle yarns can help fill-in void spaces between larger diametersingles yarns.

(h) Spinning Nanotube Yarns from Either Free-Standing or SubstrateSupported Nanotube Sheets and Ribbons

The inventors have demonstrated that nanotube sheets can be drawn from ananotube forest, separated into ribbons, and that these ribbons can besubsequently twisted to make yarns. Similarly, the inventors show thatribbons drawn from a nanotube forest can be subsequently twisted to makeyarn.

For instance, Example 36 shows a process wherein a 3 cm widefree-standing ribbon is folded upon itself along the draw direction andthen subsequently twisted to make a 50 micron diameter yarn.

Example 37 demonstrates a process wherein forest-spun nanotube sheetsare attached to a plastic film substrate, densified on this substrateusing liquid infiltration and evaporation, and then draw-twist spun intoa carbon nanotube yarn by drawing a nanotube sheet ribbon from theplastic film substrate and twisting this ribbon.

Example 52 shows that liquid-densified nanotube sheet stacks can beformed on cellulose tissue paper, that the nanotube sheets or ribbonscan be easily peeled from this cellulose substrate, and that theseribbons can be twist spun to make strong nanotube yarn. This exampleindicates that any of a variety of porous substrates, such aspoly(propylene) and polyethylene-based paper sheets, can be used ascarrier substrates for the storage and shipment of densified nanotubeyarn sheets in rolls, and the latter separation of nanotube sheets fromthe substrate as free-standing ribbons or sheets for subsequentapplications, such as the twist-based spinning of yarns, the formationof lamellar composites, and the use of ribbons and sheets as electrodes.

Polymer-infiltrated nanofibers yarns can be made by (a) placingdensified or non-densified oriented nanofiber sheets (or a singleoriented nanofiber sheet) on a fusible substrate film or film strip sothat the nanofiber sheets share a common nanofiber orientationdirection, (b) optionally cutting or otherwise sectioning the resultinglaminate along the nanofiber orientation direction to provide a laminateribbon of suitable width (determined according to the desired yarndiameter), (c) heating the ribbon laminate while the ribbon laminate isunder tension in the orientation direction or compression is appliedorthogonal to the ribbon surface, so that the fusible material attachesto and at least partially infiltrates into the nanofiber sheet, (d)optionally drawing the ribbon laminate while the fusible material is ata temperature where it is easily deformable, (e) optionally twisting theribbon while the fusible material is at a temperature where it is easilydeformable, and then (f) cooling the fusible material to ambienttemperature. The fusible substrate is preferably a fusible organicpolymer and the nanofiber sheet is preferably a carbon nanotube sheet,and the carbon nanotube sheet is preferably made by draw from a carbonnanotube forest.

In another useful process, (a) densified or undensified nanotube sheetsare laminated together with a sheet or sheets of a fusible sheetmaterial to form a laminate, (b) the laminate is heated to above atemperature at which point fusion occurs while the laminate is undertension in an orientation direction or compression orthogonal to thesheet surface, and (c) cooling the laminate to ambient temperature. Thisprocess can be optionally conducted so that fusion between the fusiblematerial and the nanotube sheets between heated rollers that provide alateral pressure. The product of this process can optionally be cut intoribbon shaped yarns that are infiltrated with the fusible material. Thefusible material is optionally and preferably an organic polymer. Forthis purpose of film cutting, micro-slitter and winder equipment isavailable from Ito Seisakusho Co., Ltd (Japan) that is suitable forconverting continuous nanotube sheets to continuous ribbons and yarns.

(i) Post Yarn Spinning and Post Ribbon and Sheet Fabrication ProcessesUsing Actinic Radiation and Thermal Treatment

Various means can be optionally employed for the post spinningprocessing of twisted yarns of Section 1(a), the false-twist yarns ofSection 1(b), the liquid-densified sheets and ribbons of Section 1(d),and the liquid-densification spun yarns of Section 1(e).

These methods include thermal annealing at a temperature of less than2500° C. for multiwalled carbon nanotubes and less than 1700° C. forsingle walled carbon nanotubes or exposure to actinic radiation, such asgamma ray, electron beam, microwave, or radiofrequency radiation.

Typically, mechanical strength increases as a result of such treatments(perhaps due to inter-tube coalescence for single walled carbonnanotubes and inter-tube covalent bonding for both single walled andmultiwalled carbon nanotubes) and then decreases with further treatment.Thermal annealing can optionally and usefully be combined eithersimultaneously of sequentially and thermal annealing can optionally beconducted by resistive heating caused by passage of an electricalcurrent through the nanotube yarns and sheets. The conditions needed tomaximize strength by such processes strongly depend upon nanotube type,but can be easily determined for a particular nanotube type and assemblytype by one having ordinary skill in the art. Information on thesemethods is found, for example, in P. M. Ajayan and F. Hanhart, NatureMaterials 3, 135 (2004), in T. J. Imholt et al., Chem. Mater. 15, 3969(2003), in A. Kis et al., Nature Materials 3, 153 (2004), and in US2004/0222081 A1 by J. M. Tour et al.

2. Synthesis and Modification of Nanofibers for Yarn, Sheet, and RibbonFabrication

MWNTs and SWNTs are optionally and especially preferred for use ininvention embodiments. Laser deposition, CVD, and the carbon-arcdischarge methods are optional and preferred methods for making thecarbon nanotubes, and these methods are well known in the literature (R.G. Ding et al., Journal of Nanoscience and Nanotechnology 1, 7 (2001)and J. Liu et al., MRS Bulletin 29, 244 (2004)). Synthetic methodsgenerally result in mixtures of nanotubes having different diameters.Use of catalyst for nanotube synthesis that is close to monodispersed insize (and stable in size at the temperatures used for synthesis) candramatically decrease the polydispersity in SWNT diameter, and nanotubeshaving this narrower range of nanotube diameters can be useful forinvention embodiments. S. M. Bachilo et al. describe such a method inJournal of the American Chemical Society 125, 11186 (2003).

The twisted SWNT yarns, nanotube sheets, and nanotube ribbons of thepresent invention can be produced from nanotube forests analogously asdescribed here for MWNT yarns. However, the preparation of SWNT forestsdiffers from that of MWNT forests. The alcohol-CVD technique is usedsuccessfully to synthesize SWNT forest (Y. Murakami et al., Chem. Phys.Lett. 385, 298 (2004)). The method of forest growth described by K. Hataet al. in Science 306, 1362 (2004) is especially useful, since both SWNTand MWNT forests result from this method and the forest height can behigher than 2.5 mm. In this method catalytic activity is stimulated by aprecisely controlled amount of water vapor during CVD growth of thenanotube forests.

As for MWNT forests, not all SWNT forests can be used advantageously forspinning yarns and for fabricating yarns and ribbons by forest-basedprocesses. For this purpose, the SWNT forest should preferably have thecharacteristics described in Section 1(f).

Multiple forest layers can readily be grown upon each other (forexample, using the methods described by X. Li et al. in Nano Letters 5,1998 (2005)) and these different layers (when optimized for forestspinning or sheet or ribbon draw) can be simultaneously utilized, whichoptimizes materials throughput. These stacked forests can optionally bestripped from the substrate prior to the sheet, ribbon, or yarnfabrication by the methods of this invention.

The nanofibers used for spinning and for ribbon and sheet fabricationcan optionally contain coiled (FIG. 14) or crimped nanofibers (FIG. 15).One benefit of such inclusion is an increase in the extensibility of thenanofiber yarns as a consequence of the increase in the effectivebreaking strain of the coiled or crimped nanofibers. One optionallypreferred method for spinning such coiled or crimped nanofibers intoyarns is to utilize draw-twist assembly from a forest comprised of suchcoiled or crimped nanofibers. An alloy of iron with indium tin oxide ascatalyst can be used to grow coiled or crimped nanotubes as forests (seeM. Zhang et. al., Jpn. J. Appl. Phys. 39, 1242 (2000)).

Various methods of separating SWNTs according to electrical propertiesare useful for invention embodiments, such as for enhancing achievedelectrical conductivity. Examples of known methods for such separationinvolve (1) use of charge transfer agents that complex most readily withmetallic nanotubes, (2) complexation with selected types of DNA, and (3)dielectrophoresis (R. Krupke et al., Nano Letters 3, 1019 (2003) and R.C. Haddon et al., MRS Bulletin 29, 252-259 (2004))

Yarn performance can also be optimized by filling component nanotubes ornanotube scrolls (single spirally-wrapped graphite sheet) with materialsto enhance mechanical, optical, magnetic, or electrical properties.Various methods are particularly useful in invention embodiments forfilling or partially filling nanotubes. These methods for SWNTs andMWNTs typically include a first step of opening nanotube ends, which isconveniently accomplished using gas phase oxidants, other oxidants (likeoxidizing acids), or mechanical cutting. The opened nanotubes (as wellas scrolled nanotubes) can be filled in various ways, like vapor, liquidphase, melt phase, or supercritical phase transport into the nanotube.Methods for filling nanotubes with metal oxides, metal halides, andrelated materials can be like those used in the prior art to fill carbonnanotubes with mixtures of KCl and UCl₄; KI; mixtures of AgCl and eitherAgBr or AgI; CdCl₂; CdI₂; ThCl₄; LnCl₃; ZrCl₃; ZrCl₄, MoCl₃, FeCl₃, andSb₂O₃. In an optional additional step, the thereby filled (or partiallyfilled) nanotubes can be optionally treated to transform the materialinside the nanotube, such as by chemical reduction or thermal pyrolysisof a metal salt to produce a metal, such as Ru, Bi, Au, Pt, Pd, and Ag.M. Monthioux has provided (Carbon 40, 1809-1823 (2002)) a useful reviewof these methods for filling and partially filling nanotubes, includingthe filling of nanotubes during nanotube synthesis. The partial orcomplete filling of various other materials useful for inventionembodiments is described in J. Sloan et al., J. Materials Chemistry 7,1089-1095 (1997).

Nanofibers need not contain carbon in order to be useful for inventionembodiments, and a host of processes are well known in the art formaking nanofibers that are not carbon based. Some examples are thegrowth of superconducting MgB₂ nanowires by the reaction of singlecrystal B nanowires with the vapor of Mg (Y. Wu et al., AdvancedMaterials 13, 1487 (2001)), the growth of superconducting lead nanowiresby the thermal decomposition of lead acetate in ethylene glycol (Y. Wuet al., Nano Letters 3, 1163-1166 (2003)), the solution phase growth ofselenium nanowires from colloidal particles (B. Gates et al., J. Am.Chem. Soc. 122, 12582-12583 (2000) and B. T. Mayer et al., Chemistry ofMaterials 15, 3852-3858 (2003)), and the synthesis of lead nanowires bytemplating lead within channels in porous membranes or steps on siliconsubstrates. The latter methods and various other methods of producingmetal and semiconducting nanowires of types suitable for the practice ofinvention embodiments are described in Wu et al., Nano Letters 3,1163-1166 (2003), and are elaborated in associated references. Y. Li etal. (J. Am. Chem. Soc. 123, 9904-9905 (2001)) has shown how to makebismuth nanotubes. Also, X. Duan and C. M. Lieber (Advanced Materials12, 298-302 (2000)) have shown that bulk quantities of semiconductornanofibers having high purity can be made using laser-assisted catalyticgrowth. These obtained nanofibers are especially useful for inventionembodiments and include single crystal nanofibers of binary group III-Velements (GaAs, GaP, InAs, InP), tertiary III-V materials (GaAs/P,InAs/P), binary II-VI compounds (ZnS, ZnSe, CdS, and CdSe), and binarySiGe alloys. Si nanofibers, and doped Si nanofibers, are also useful forinvention embodiments. The preparation of Si nanofibers by laserablation is described by B. Li et al. (Phys. Rev. B 59, 1645-1648(1999)). Various methods for making nanotubes of a host of usefulmaterials are described by R. Tenne in Angew. Chem. Int. Ed. 42,5124-5132 (2003). Also, nanotubes of GaN can be usefully made byepitaxial growth of thin GaN layers on ZnO nanowires, followed by theremoval of the ZnO (see J. Goldberger et al., Nature 422, 599-602(2003)). Nanofibers having approximate composition MoS_(9-x)I_(x), whichare commercially available from Mo6 (Teslova 30, 1000 Ljubljana,Slovenia) are included in preferred compositions (most especially for xbetween about 4.5 and 6).

While some of these above-described non-carbon-based nanofibers do nothave the dimensional characteristics that are optionally preferred fornanofiber spinning and the fabrication of nanofiber sheets and ribbons,the prior art teaches methods for synthesizing nanofibers of these typesthat are in optionally preferable dimensional ranges. For example,synthesis of nanofibers by templating anodized alumina is a well knowntechnology, and the nanofiber diameter and nanofiber length can beappropriately adjusted by appropriate selection of the thickness of theanodized alumina and the diameter of the channels in this anodizedalumina.

Nanoscrolls are especially useful for invention embodiments, because theinventors find that they can provide mechanical properties advantagesover multiwalled nanotubes and other non-scrolled nanofiber types. Thesenanoscrolls are individual sheets or a thin stack of sheets of a layeredmaterial that automatically wind to make a scroll, which is structurallyanalogous to a jelly roll. Almost any sheet-like material canself-assemble into scrolls—as long as the lateral sheet dimension issufficiently large that the energy gain from non-covalent bindingbetween layers of the scroll can compensate for the elastic energy costof forming the scroll. Some examples of materials that have been shownto form nanoscrolls are bismuth, BN, C, V₂O₅, H₂Ti₃O₇, gallium oxidehydroxide, zinc and titanium oxides, CdSe, Cu(OH)₂, selectedperovskites, InGa/GaAs and Ge_(x)Si_(1-x)/Si heterolayer structures, andmixed layer compounds like MTS₃ and MT₂S₅ (M=Sn, Pb, Bi, etc.; T=Nb, Ta,etc.). This generality of the scroll formation process for layeredmaterials, from bismuth to carbon and boron nitride, means that there isa host of candidate compositions to choose from for yarn formation.Since scrolls can be made by simply exfoliating materials that arepresently made in high volume at low cost, yarns of this invention canalso be made at low cost. Methods of synthesizing nanoscrolls of a hostof layered materials are known, and these methods can be used for thepractice of present invention embodiments (see L. M Viculis, L. M., J.J. Mack, and R. B. Kaner, Science 299, 1361-1361 (2003); Z. L. Wang,Advanced Materials 15, 432-436 (2003); X. D. Wang et al., AdvancedMaterials 14, 1732-(2002); W. L. Hughes and Z. L. Wang, Applied PhysicsLetters 82, 2886-2888 (2003); J. W. Liu et al., Journal of PhysicalChemistry B 107, 6329-6332 (2003); and Y. B. Li, Y. Bando, and D.Golberg, Chemical Physics Letters 375, 102-105 (2003)).

3. Non-Forest Nanofiber Assemblies Suitable for Yarn, Sheet, and RibbonFabrication

As an alternative to employing a nanotube forest as the pre-primary orprimary assembly, various other nanofiber arrays can be employed.

For instance, Example 37 describes a method of twist spinning of carbonnanotube yarns from a densified nanotube sheet. In this example thedensified nanotube sheet serves as a pre-primary array. An as-drawn,free-standing MWNT sheet (made as in Example 21) was placed onto asubstrate (e.g., glass, plastic, or metal foil) and densified using aliquid. A plastic substrate, like Mylar film, was most convenientlyused. A desired width of the densified sheet was easily drawn from thesubstrate using an adhesive tape to start the draw process (in which thenanofiber yarn of ribbon is generated by peeling nanotubes from thesubstrate). By attaching one end of the separated sheet strip to a motorto introduce twist while the yarn was drawn, a uniform diameter spunyarn was obtained.

Example 36 demonstrates that the solid-state fabricated MWNT sheets canbe conveniently drawn and spun into large diameter yarns having uniformdiameter. In this example the densified nanotube sheet serves as aprimary array. A 10.5 cm long, 3-cm-wide as-drawn nanotube sheet wasfolded upon itself to make a quasi-circular assembly having about thesame length. One end was attached to the tip of a spindle and the otherend was attached to a fixed cupper wire. By introducing twist, uniformspun yarn was formed at a twist level of ˜2000 turns/meter.

Although an aerogel comprising sufficiently long carbon nanotubes issuitable for twist-based spinning of yarns, the benefits of lateralstress transfer are not realized unless the ratio of nanofiber length tonanofiber circumference is above 5 and more preferably above 20. Whilenanotube yarns have been previously spun in a CVD furnace at about 100°C. and either simultaneously or subsequently twisted, the describedexperiments (Y. Li et al., Science 304, 276 (2004), I. A. Kinlock et.al., WO 2005/007926 A2, and M. Motta, Nano Letters 5, 1529 (2005))provided a ratio of nanofiber length to yarn circumference of belowunity, which is inadequate for achieving the benefits of twist-generatedlateral stress transfer.

Carbon nanotube aerogel comprised of suitably long nanotubes can be usedas a pre-primary array for false-twist-based yarn spinning and forliquid-densification-based yarn spinning, as well as for the productionof strong ribbons and sheets by the liquid-densification-basedstrengthening of aerogel ribbons or sheets drawn from the aerogel. Drawnribbons of these aerogels, optionally strengthened by liquiddensification, can be converted into nanotube yarns using twist, falsetwist, or a combination of false twist and twist.

Other nanofiber aerogels are also suitable as a pre-primary array forfalse-twist-based yarn spinning and for liquid-densification-based yarnspinning, as well as for the production of strong ribbons and sheets bythe liquid-densification-based strengthening of aerogel ribbons orsheets drawn from the aerogel. Drawn ribbons of these aerogels,optionally strengthened by liquid densification, can also be convertedusing twist, false twist, or a combination of false twist and twist intonanofiber yarns. Examples of nanofiber gels providing usefulcompositions for spinning and for sheet and ribbon formation arevanadium oxide aerogels, vanadium oxide/carbon nanotubes compositesaerogels, and nanofibrillar cellulose aerogels. The preparation of gelsof these types are described by J. S. Sakamato and B. Dunn (Journal ofthe Electrochemical Society 149, A26 (2002)), by W. Dong et al. (Scienceand Technology of Advanced Materials 4, 3 (2003)), and by H. Jin et al.(Colloids and Surfaces A 240, 63 (2004)). Well-known methods can beusefully employed for increasing nanofiber length and minimizing thelength-to-width ratio, so as to improve or provide applicability ofthese nanofibers for fiber spinning and for sheet and ribbon draw.

Magnetically oriented nanofiber sheets, electrically oriented nanofibersheets, or nanofiber sheets oriented by shear flow can be employed asprimary arrays for yarn spinning. These nanotube sheets can be obtainedin highly oriented form by various processes, such as the application ofeither shear flow fields or magnetic or electric fields during afiltration process that provides nanotube sheets (see M. J. Casavant etal., J. Applied Physics 93, 2153-2156, (2003)), and such forms areuseful for the practice of invention embodiments.

In order to remove impurities, it is optionally useful for someapplications to anneal the nanofiber sheets to remove impurities, suchas possible surfactants used for nanotube suspension and possiblefunctionalities introduced during nanotube purification. This optionalannealing for carbon nanotubes is preferably conducted at temperaturesof at least 400° C. for 0.5 hours or longer. In order to preserve carbonnanotube structure, annealing is optionally and preferably carried outin an inert atmosphere at a temperature that is preferably below about1500° C. for single walled nanotubes.

Such oriented arrays of nanofibers can be employed as a primary assemblyfor twist-based spinning. The inventors have discovered that suchtwist-based spinning will not be very successful in producing a highstrength yarn unless the maximum total applied twist in one directionper unit fiber length for a twisted yarn of diameter D is at leastapproximately 0.06/D turns and a significant component of the nanofibershave (i) a maximum width of less than approximately 500 nm, (ii) aminimum length-to-width ratio of at least approximately 100, and (iii) aratio of nanofiber length to yarn circumference greater thanapproximately 5.

The inventors find that the oriented carbon nanotube sheets described inthe literature (having nanofiber lengths of less than a few microns) arelargely unsuitable for making high strength twisted yarns of diameter 1micron or larger. The reason that the inventors find is that thenanofiber length should preferably be at least five times the yarncircumference and more preferably 20 times the yarn circumference.However, the inventors find that the filtration-based sheet formationprocess works for much longer nanotubes (such as the up to 300 micronlong nanotubes produces by the method of Example 1), and such longernanotubes are optionally and preferably employed for inventionembodiments.

Yarns made by coagulation spinning processes are also useful forinvention embodiments as long as nanofiber lengths in these yarns areincreased to preferably over at least five times the circumference ofthe yarn. This condition has not been realized for the spun yarns in theliterature. Moreover, the nanotube lengths in these yarns are morepreferably over at least twenty times the circumference of the yarnThese coagulation-based spinning methods include, for example, thecoagulation spinning using polymers like polyvinyl alcohol in thecoagulation bath (B. Vigolo et al., Science 290, 1331 (2000); R. H.Baughman, Science 290, 1310 (2000); B. Vigolo et al., Applied PhysicsLetters 81, 1210 (2002); and A. B. Dalton et al. Nature 423, 703(2003)), coagulation spinning using an aqueous nanotube dispersion andan acidic or basic non-polymeric coagulant (co-pending PCT PatentApplication Serial No. PCT/US2005/035220), and spinning processes thatuse acidic spinning solutions and a non-polymeric coagulation bath (V.A. Davis et al., U.S. Patent Application Publication No. 20030170166).However, for those spinning processes in which a polymer coagulant isused or the spinning solution contains a polymer, the inventors findthat the polymer-containing yarn is preferably drawn to a highlyoriented state while the polymer is present and that the polymer is thenmost desirably substantially removed (such as by thermal pyrolysis)prior to the twist process. In the invention embodiments of thisparagraph the yarn formed by coagulation spinning can be the primaryassembly for the spinning process. The inventors also find that hollowyarns containing highly oriented nanofibers are suitable for thepractice of invention embodiments involving twist processes, providedthat the shortest nanofiber lengths in the hollow yarn are greater thanapproximately five times the yarn circumference.

Nanotube yarns spun from super acids (V. A. Davis et al., U.S. PatentApplication Publication No. 20030170166; W. Zhou, et al., J. AppliedPhysics 95, 649-655 (2004)) are also optionally especially preferred asprimary assemblies for preparation of twisted yarns of inventionembodiments. However, the nanotubes in the yarns that have been spun inthe prior art using the super acid spinning process are too short forderiving high performance carbon nanotube yarns having micron and largerdiameters, unless a polymer binder is employed. Also, the reported yarndiameters are about 60 μm and larger (W. Zhou, et al., J. AppliedPhysics 95, 649-655 (2004)). Hence, both because of the large spun yarndiameters and the short nanotube lengths, the insertion of twist doesnot improve the performance of these yarns, either as-spun or afterthermal annealing. According to the teachings of the present inventionembodiments, twist spinning or draw-twist spinning should preferablyprovide a ratio of nanofiber length to yarn circumference that isgreater than approximately 5 for a significant component of nanotubes inthe yarn. More preferably, a significant component of nanofibers in theyarns has a minimum ratio of nanofiber length to yarn circumference ofgreater than approximately 20. Optionally and most preferably, a majorcomponent of the nanofibers has a ratio of nanofiber length to yarncircumference of greater than approximately 20. Hence, the acid-spunyarns of the prior art with 60 μm yarn diameter can be twisted tosubstantially achieve the benefits of twist if the nanofiber length isabout 940 μm, which is about 1000 times the likely nanofiber length inthe super acid spun fibers of the prior art.

4. Twist and False-Twist Polymer-Free Nanofiber Yarns Having PreviouslyUnobtainable Diameters

The invention embodiments provide twisted yarns that are a thousand-foldor smaller in diameter than the twisted yarns of the prior art. Aboutone hundred thousand individual carbon nanofibers are in thecross-section of a 5 μm diameter nanotube yarn, corresponding to ananofiber density of 5000 nanofibers per square micron, as compared withthe 40-100 fibers in the cross-section of typical commercial wool(worsted) and cotton yarns. For comparison, the smallest diameternanotube fibers reported using prior-art technologies are more than tentimes larger in diameter than the micron diameter twisted carbonnanotube yarns whose preparation is described in Example 2. Since yarnvolume per meter of yarn length and linear density are proportional tofiber diameter squared, the yarns of present invention embodiments aremore than one hundred times lower in linear density and volume per meterthan has been achieved for textile yarns in the prior art. Microfibers,which are widely used because they are extremely soft to the touch,drapable, and highly absorbent, are defined in the textile industry asbeing of under one denier (i.e., less than 1 den), that is one filamentweighs less than one gram per nine thousand meters of length or 0.11mg/m. For comparison, the linear density of unplied MWNT yarns of 5micron diameter (called “singles” and comprising about 100,000 fibers inthe cross section) is typically about 10 μg/m (0.09 den) compared withthe usual 10 mg/m (90 den) and 20-100 mg/m (180-900 den) for cotton andwool yarns, respectively.

As a result of the small diameters of the twisted nanofiber yarns andthe component nanofibers, linear densities are achieved that are smallerthan current twisted microfiber yarns by factors of between 10² and 10⁶,which makes the yarns promising for textile applications in militaryclothing and space suits. Benefits in textiles include the combinationof “breathability” and water and wind resistance, impermeability ofclosely-woven micron-diameter yarns to bacteria like anthrax, highthermal and electrical conductivities, radio-frequency and microwaveabsorption, electrostatic discharge protection, penetration protection,and fabric softness and drapability, which is in stark contrast with theuncomfortable stiffness of some electronic textiles. Comparabletoughness to Kevlar® fibers used in antiballistic vests, resistance toknot and abrasion-induced failure, high failure strains, and highultraviolet light and thermal stability are other major advantages ofthe nanotube yarns for textile applications.

As a result of these achieved benefits, for some application areas, theoptionally preferred yarns of invention embodiments for these low denierapplications areas have a nanofiber singles yarn diameter of less thanapproximately 10 microns and a yarn length of over one meter. Optionallymore preferred for these applications, the nanofiber singles yarndiameter is less than approximately 5 microns. The draw-twist yarnsoptionally and preferably contain at least 500 nanofibers that passthrough each square micron of yarn cross-sectional area. Optionally andmore preferably, at least 1000 nanofibers pass through each squaremicron of nanofiber yarn cross-sectional area.

5. Elaboration on Twist Insertion, Densification, and Filament StorageMethods During Spinning

Various known methods of twist insertion can be used for introducingtwist during nanotube spinning into yarns. Such methods include, but arenot limited to, ring spinning, mule spinning, cap spinning, open-endspinning, vortex spinning, and false twist spinning technologies (see E.Oxtoby, Spun Yarn Technology, Butterworths, 1987 and C. A. Lawrence,Fundamentals of Spun Yarn Technology, CRC Press, 2002). Mule spinninghas the disadvantage of being a batch process (spin then wind-on), buthas the advantage of not requiring rings or travelers.

A novel continuous spinning apparatus is provided for spinning fine andultra-fine nanofiber yarns, which introduces twist as it winds the spunyarn onto a bobbin. The apparatus is shown schematically in FIG. 38. Thefiber source is the nanotube forest on substrate (3801). The producedyarn (3802) is passed through an initial yarn guide (3803). The spinningapparatus comprises a spindle (3805), a donut-shaped winding disk (3806)with an associated winding yarn guide (3804), an electromagnet (3807),and a donut-shaped metal magnetic disk (3808), which contacts theferromagnetic spindle base (3805), which is typically made of steel. Onone end, the spindle is driven by a variable-speed motor (not shown). Onthe opposite end there is a removable bobbin (3811) that takes up andstores the spun fiber. The spindle base contains a spindle pin (3810)that protrudes from the spindle base, and passes through the centers ofthe magnetic disk (3808) and the ferromagnetic winding disk (3806). Avariable-speed motor rotates the spindle at angular speed ω, and themagnetically-induced friction between the spindle base (3809) and themagnetic disk (3808) and between the magnetic disk and the ferromagneticwinding disk (3806) causes the winding disk to rotate. An electromagnetis used to introduce a variable braking force onto the winding disk,which reduces its angular speed (ω₂) relative to the spindle. Therotation of the drafted nanofiber assembly about the axis of the spindleintroduces twist, thereby forming the yarn, while the slower rotation ofthe winding disk winds the spun yarn onto the spindle. The winding speedis determined by the speed difference between ω₁ and ω₂[ω(winding)=ω₁−ω₂], which can be continuously adjusted by varying thevoltage applied to the electromagnet. Advantageously, both twist leveland spinning speed can be independently controlled by an electronicinterface to independently regulate motor speed and applied magneticfield. This system imposes minimal tension to the spun yarns and canhandle spinning of yarns with either high or low breaking force. Thissame apparatus can also be utilized to ply multiple single-strand yarnstogether to continuously make multi-strand yarns. In such a case, thenanotube forest is replaced by reels of unplied yarn.

Various modifications of the spinning apparatus can be usefullyemployed. For example, the magnetic disk (3808) can be eliminated, and adirect frictional force between the winding disk and the spindle basecan be provided by spring loading the winding disk. The electromagnet(3807) can be eliminated if spring loading is provided by electricallycontrolled actuators (such as ferroelectric or ferroelectric actuators),Alternatively, the magnetic disk (3807) can be replaced with aferroelectric disk whose thickness is electrically controlled toregulate indirect mechanical coupling between the winding disk and thespindle base. In this latter case, the electromagnet (3807) can beeliminated and frictional forces between the spindle base, theferroelectric disk, and the winding disk can be produced by springloading.

Another preferred method of spinning the carbon nanotubes into yarns isto employ a direct spinning method that twists as it winds the spun yarnonto a bobbin. The apparatus is outlined schematically in FIGS. 19 and20.

FIG. 19 shows a spinning unit (1900) consisting of a substrate cradle(1901) located above a bobbin (1902) that rotates about an axis (1903)that is coincident with the yarn axis, while simultaneously rotatingabout a take-up axis (1904). The rotation about the yarn axis introducestwist (1905) into the drafted nanotube assembly (1906), thereby formingthe yarn, while the rotation about its own axis winds the spun yarn ontothe bobbin fitted firmly onto the drive roller.

FIG. 20 gives the details of the substrate cradle, which is shown with 6substrate units (2001) supported by a network of substrate-holder arms(2002), a central shaft (2003), and cross-supports (2004). It will beappreciated, however, that many variations of the design are possiblethat are consistent with the outlined scheme. It is also possible thatMWNT forests be grown on one side or both sides (2005) of the substrate.An advantage of growing nanotubes on both sides of the substrate is thatit increases the capacity of the spinner.

The advantages of this direct spinning method are that the coincidencebetween the spinning and yarn axes eliminates the spinning balloon andthe nanotube yarn does not make contact with any surfaces until afterthe twist is inserted, by which time it has sufficient cohesion to behandled without disruption. The twist level can be set independently ofthe wind-on by running the wrap and twist drives independently of eachother using variable speed motors.

The apparatus of FIGS. 19 and 20 can be made continuous by usingsubstrates for the nanotube forests that are flexible belts, which bendaway from the yarn produced by nanotube extraction, continuously move toa furnace for nanotube growth, and then return to the point of nanotubeextraction to produce yarn.

Technologies known in the art can be fitted to the spinner to improvefunctionality and productivity, such as monitoring of the tension in theyarn, automatic loading of new substrates and removal of usedsubstrates, automatic threading-up and doffing, and systems to managethe build of the yarn package.

The inventors have surprisingly discovered that a variant of open end orbreak spinning can be downsized by a factor of over a thousand, fromapplicability to micron diameter fibers to nanometer diameter fibers.Open end spinning of conventional textile fibers comprises the followingsteps: (1) preparation of an assembly of straight, parallel, andindividualized fibers; (2) a method for detaching single fibers from theassembly; (3) means of conveying the fibers to the internal surface of acup-shaped collector (rotor); (4) methods for supporting and driving thecollector at high speed; and (5) a method of withdrawing the fibers fromthe collector, during which twist is inserted to form the yarn, andwinding the yarn onto a package. All of these steps are also required toopen end spin nanofibers, but the inventors have found adaptations areneeded in order for the process to operate satisfactorily for nanoscalefibers, namely, (1) ensuring that all surfaces that the nanofibers comeinto contact with and the surfaces of the nanotubes are chosen toprevent the nanofibers from sticking to the surfaces and (2) developingand using methods applicable on the nanoscale for delivery of nanofibersthat are suitably individualized and oriented for delivery to the to thecollector for twist insertion. Unlike the case of conventional spinning,the suitably individualized nanofibers can be nanofiber bundles or robescomprising many thousands for component nanofibers that must beappropriately assembled during the steps of yarn spinning or duringpre-spinning processing. While natural fibers also contain componentnanofibers, nature's biology does the self assembly.

The design of an open end spinner suitable for spinning nanofibers isshown in FIG. 43. The diagram shows nanotubes being detached from asupply package (4301) on which they were deposited by sheet-drawing froma pre-primary array. Roller 4301 may be quite long in order to maximizestorage. Nanofibers and nanofiber assemblies are detached from thesupply roller by means of a high speed beater (4302) that is equippedwith a multitude of fine pins of high surface finish located in closeproximity to the supply roller. The nanofibers and nanofiber assembliesare ejected into a transport tube (4303) where airflow carries thenanofibers into a rotor 4304. The airflow is generated by sustaining theair pressure in the rotor below atmospheric pressure. The nanofibers andnanofiber assemblies collect against the internal face of the rotor andslide into a groove (4305) under the influence of the centrifugal forcewhere they form an assembly of largely parallel fibers. Once asufficient number of nanofibers have accumulated in the groove, a seedyarn (4306) is introduced into the rotor, which is aided by the lowpressure, whereupon the nanofibers start to twist to form a yarn, oneturn of twist being introduced for every turn of the rotor. Immediately,the yarn is withdrawn by the rollers 4307 but as with conventional openend spinning, a doffing tube navel (4308) is used to insert false twistinto the forming yarn in the rotor in order to increase the twist in thetail (4306) and improve the reliability of spinning. As the yarn isformed, it is wrapped onto a yarn package 4309 by a package winder 4310.Variations of the basic design are possible as would be known by thoseskilled in the art.

False twist spinning is used in conventional textile processing eitherto impart bulk to continuous filaments or for staple fibers when asecond yarn is available to trap the false twist in a twofold structure.False twist spinning has limited use for conventional staple fiber yarnsbecause the twist disappears once the yarn passes the twister and theyarn loses its strength and tenacity. Surprisingly, the inventors findthat the nanofiber yarns described here retain strength and tenacityafter introduced twist is removed, which is likely due in part todensification of the yarns. This means that false twist can be used toproduce yarns that are suitable for use either with or without appliedinter-yarn binder, such as an infiltrated polymer. This discoveryprovides the motivation for the false-twist spinning apparatus describedin FIGS. 44-46. Even if the nanofiber yarns are later subjected to apermanently introduced twist, the introduced false twist increasesnanofiber strength so that higher processing speeds are possible withoutcausing yarn breakage.

These measurement results on the effects of twist insertion andequivalent twist de-insertion (called false twist insertion) areprovided in Example 40. The obtained strength (when twist insertion isfollowed by equal twist de-insertion) (113 MPa) is much higher than thenegligible strength for non-twisted yarns, but lower than for the casewhere the initially inserted degree of twist insertion is retained (339MPa). Nevertheless, the yarns that have undergone the twistinsertion/twist de-insertion process are highly desirable for use informing nanotube/polymer composite yarns and have both high strength andhigh toughness, as well as for developing the dense packing thattransfers stress as a result of van der Waals interactions between verylong nanotubes. The point here is that appropriately laterally couplednanofiber yarns will have the greatest strength when the twist is zero.Twist is introduced to provide lateral coupling. However, theinfiltration of nanofiber yarns with a coupling agent (such as aninfiltrated polymer) can also provide the needed lateral coupling. Inaddition, any false twist process that provides densification of yarns(and corresponding adequate enhancement of nanofiber-nanofiber coupling)will provide high strengths if the nanotubes are adequately long.

Corresponding to these discoveries, invention embodiments are providedin which false twist is introduced and the nanofiber yarn is laterinfiltrated with a binding agent, such as a polymer. The nature ofappropriate binding agents is diverse and can include polymers, metals(such as those melt infiltrated, chemically, or electrochemicallyinfiltrated), and other organic and inorganic materials (such as SiO₂).Binding agents, such as a polyacrylonitrile, can be optionallypyrolized, and additional infiltration steps can be followed byadditional pyrolysis steps in order to obtain optimal filling of thenanofiber yarns with binding agent.

False twist spinning can also be used to easily provide a high twistzone that can densify the yarn at high speed as a pretreatment toinserting lower levels of real twist. Without the preliminarydensification, higher levels of real twist would be required slowing theproduction rate of the yarn. Apparatus to do this is shown in FIG. 46,where the pre-primary array (4601) is shown being drawn off a substrateand being twisted by the false twist spinneret 4602 to give a highlytwisted yarn 4603 upstream of the spinneret. Downstream of the spinneretis a conventional spinning system 4604, comprising a bobbin for windingthe yarn (4605), a traveler-type hook (4606) for inserting twist and adrive (4607) that introduces a lower level of twist than in the falsetwist zone. Motions to provide for package build are not shown but arefamiliar to those skilled in the art. Because the yarn has beendensified by the high twist imparting some strength, lower levels ofreal twist are required in order to attain reasonable strengths for thenanofiber yarn.

A design for a false twist system that is suitable for spinningnanofibers is shown in FIG. 44. Nanofibers are withdrawn from thenanotube forest (4401), which is supported on substrate (4402), and arebeing simultaneously twisted by the false-twist spindle 4403 (spinneret)to form the spinning triangle (4404) and the highly twisted yarn (4405).When the spinning system has reached equilibrium, the yarn leaving thespinneret has no twist (4406) but has some strength because ofdensification that was imparted as a result of the twist upstream of thespinneret. The yarn is wound onto a package (4407) by the package winder(4408).

The spinneret (4403) is shown in greater detail in FIG. 45. Thespinneret 4501 comprises a cylindrical tube (4502), a section 4503 forlocating a supporting bearing, and a pulley (4504) for driving thespinneret at high speed. Two toroidal ceramic yarn guides (4505 and4506) are mounted at the opposite end of the cylinder 4502 to the pulleyto support the yarn during passage through the spinneret. A hole (4507)with an appropriate cross-sectional shape is bored through the body ofthe cylinder 4502 perpendicular to the shaft. A ceramic pin 4508 isprovided around which the yarn can be looped to provide effectively acrank for inserting twist. The pin is shaped to provide positivelocation of the yarn above the axis and generally uses a ‘U’ saddleshape in the plane containing the axis of the spinneret. An additionalrefinement is to locate a narrow waist on the pin located over the axisof the spinneret. The pin is open at one end to provide for easythreading up of the spinneret.

Another method of continuous spinning carbon nanotubes and othernanofibers into yarns is to employ a direct spinning method that twistsas it winds the spun yarn onto a bobbin. The apparatus is outlinedschematically in FIG. 106. The fiber source is the nanotube forest onsubstrate (10601), though other nanofiber sources can be used. Theproduced yarn (10602) is passed through an initial yarn guide (10603).The spinning apparatus comprises a spindle (10605), a winding disk(10606) with an associated winding yarn guide (10604), a spinning motor(10607), and a winding motor (10608), which drives the winding diskthrough a belt (10609). On one end, the spindle (10605) is driven by avariable-speed motor (10607); on the opposite end there is a removablebobbin (10611) that takes up and stores the spun yarn. The spindle(10605) is attached to motor (10607) and the spindle pin (10610) passesthrough the centers of the winding disk (10606). A variable-speed motor(10607) rotates the spindle at angular speed ω₁ and the winding motor(10608) rotates the winding disk at angular speed ω₂. The rotation ofthe drafted nanofiber assembly about the axis of the spindle introducestwist, thereby forming the yarn, while the faster rotation of thewinding disk winds the spun yarn onto the bobbin. The winding speed isdetermined by the speed difference between ω₁ and ω₂[ω(winding)=ω₂−ω₁],which can be continuously adjusted by varying the speed differencebetween the two motors. Advantageously, both twist level and spinningspeed can be independently controlled by electronic interfaces toindependently regulate motor speeds. This same apparatus can also beutilized to ply multiple single-strand yarns together to continuouslymake multi-strand yarns. In such case the nanotube forest is replaced byreels of unplied yarn or yarn having lower ply than desired in aproduct. Though not shown, provision can be added to the spinningapparatus of FIG. 106 to move the bobbin back and forth to collect theyarn.

Since surprising yarn, sheet, and ribbon strength enhancements resultfrom liquid infiltration and subsequent evaporation, the inventors usethese enhancements for processing steps. Example 38 shows the dramaticincreases in yarn strength that result from infiltration of a suitablevolatile liquid in the yarn and subsequent evaporation of this liquid.Tenacity also increases. These effects are apparently due to yarndensification due to evaporation of the volatile liquid. If no twist isapplied and the yarns are used as drawn from the forest, the yarnmechanical strength was too low to be measured using our apparatus.

The most suitable liquids for such densification and improvements inmechanical strength and tenacity are those that have sufficiently lowviscosity for penetration in nanofiber yarns, sheets, or ribbons and theability to at wet the nanotubes. While the liquid used in Example 38 isethanol, like volatile liquids having low viscosity and cohesive energydensities approximately matched to the nanofibers are also useful.

There are many systems in textile or sheet processing for adding liquidsto yarns, such as spraying, padding, and vapor coating. All of thesetechniques can be used during spinning or sheet fabrication in order toobtain strength enhancements of the spun yarn or drawn sheets. A syringepump is employed in FIG. 44, a solvent bath is used in Example 53, andcondensation of a vapor is used in Example 54.

6. Storage of Ultra-Thin Drawn Nanofiber Sheets

Ultra-thin carbon nanotube sheets can be optionally drawn and thenapplied for device construction without the necessity of storage.However, in some cases it is desirable to fabricate rolls of such sheetand to subsequently apply these rolls for applications, such as deviceconstruction.

Carbon nanotube (CNT) sheets can be drawn from a forest, attached to asubstrate film (such as a plastic, metal foil, porous paper, or Teflonfilm), densified, and wound onto a mandrel. Demonstration of thefeasibility of this process for adhesive-free, adhesive-coated, andelastomeric, and porous substrates is provided in Examples 23, 31, 32,and 45, respectively. FIG. 53 and FIG. 54 show schematic illustrationsof such processes.

Example 45 shows that carbon nanotube sheets can be deposited on acontoured surface and densified on this surface, so that the shape ofthe contoured surface is retained in the shape of the nanotube sheetarray. This mandrel can be a contoured storage mandrel. This applicationdemonstration enables, for example, the deposition of carbon nanotubesheets as a layer in a contoured composite (such as an aircraft panel),as a contoured heating element for de-icing on an air vehicle, or acontoured supercapacitor that provides both an energy storage andstructural component for a contoured car panel.

Element 5302 in FIG. 53 is a nanotube forest prepared as described inExample 1. Element 5301 is a growth substrate, element 5303 is ananotube sheet drawn from the forest, element 5304 is the substratefilm, and element 5305 is nanotube sheet attached to the substrate film.The attached nanotube sheet is densified using a liquid (element 5306),dried by a heater (element 5307), and then wound onto a mandrel. Here,rollers (two) are represented by open circles and mandrels (three) arerepresented by filled circles. By repeating the process, multilayer ofnanotube sheets can be applied to the substrate film. A variation of theprocess is illustrated in FIG. 54. Instead of using liquid, liquid vapor(element 5406) is used to densify the collected sheet and the densifiedsheet (element 5407) is wound onto a mandrel. The elements are nanotubeforest substrate (5401), nanotube forest (5402), CNT sheet (5403),substrate film (5404), CNT sheet attached to substrate film (5405), aheating system for delivery of vapor (5406), densified CNT sheet onsubstrate film (5407), substrate film delivery mandrel (5408), rollerfor consolidation of nanotube sheet and substrate film (5409), andcollection mandrel (5410). Each of the rollers in FIGS. 53 and 54 canoptionally be replaced by pairs of rollers, one on each side oflaminated nanotube sheet and substrate film.

Importantly, the densified nanotube sheet produced by the apparatus ofFIGS. 53 and 54 can be later unwound from the mandrel and separated fromthe substrate film for the twist-based spinning of yarn (see Example 37)to form free-standing densified sheets or for mechanical transfer ofselected portions of nanotube sheets to other substrates (see Example34). Also, the substrate can be an elastomeric film (or textile) that isstretched prior to attachment of the nanotube sheet (see example 32) oran adhesive coated substrate sheet (see Example 31). The stretching canbe accomplished by controlling the relative rotation rates of substratedelivery and substrate film/nanotube sheet take-up mandrels and rollers(or roller pairs) between these mandrels.

Example 50 demonstrates that nanotube sheets can be deposited on asubstrate, densified using by the liquid infiltration method, and thenpeeled from the substrate to provide a free-standing, densified sheetarray. The importance of this demonstration is that it enables thestorage of densified nanotube sheets on a mandrel, and subsequentretrieval of these densified sheets from the sheet substrate (typicallya plastic film carrier) for applications. Either three, five, or eightlayers of as-drawn, free-standing MWNT sheet (made as in Example 21)were placed onto a substrate (e.g., glass, plastic, or metal foil) anddensified using a liquid (using a process of Example 23). A plasticcarrier substrate, like Mylar film, was most conveniently used. Adesired width (or the entire width) of the densified sheet was easilypeeled from the substrate using an adhesive tape to start the sheetremoval process. Unless the densified sheet thickness is greater thanthe 30 to 50 nm thickness obtained by liquid infiltration of the sheetsmade in Example 21 (for example, as a result of using a higher forestfor the sheet draw), it is preferable to deposit a stack of more thanone sheet on the carrier substrate, since a single 30-50 nm densifiedsheet can be easily damaged during removal from the substrate.

Example 37 shows that very thin densified carbon nanotube sheets stacks(less than 150 nm in thickness) can be rolled onto a mandrel for storageand possible shipment, and then subsequently unrolled for applicationwithout separating or supporting the nanotube sheets with a carriersheet (like in the Mylar film in Example 50).

7. Chemical and Physical Modifications Before and after Fabrication ofYarns, Sheets, and Ribbons

A variety of methods can be usefully employed in invention embodimentsfor the modification of nanofibers either before or after draw-twistspinning or sheet drawing. Various benefits can result from suchmodification, such as optimization of inter-fiber friction for twistspinning, the development of inter-fiber covalent bonding for eitheryarns or ribbons, and the electrical insulation of electricallyconducting nanofiber yarns (such as by a post-spinning chemicalderivatization process for MWNTs). Chemical derivatization, physicalderivatization, surface coating, or dopant insertion can be practicedbefore or after spinning, or even during draw-twist spinning processesor after fabrication of the draw-twist yarns into articles or precursorsto articles, like woven textiles. An especially preferred method formodifying carbon nanotubes while in nanotube forests is by gas phasereactions, plasma-induced reactions, or reactions and fluid extractionaccomplished in supercritical phases, since these methods generallybetter preserve nanotube alignment within the nanotube forest than dosolution or melt phase methods. Fluorination of carbon nanotubes withfluorine gas and plasma induced surface derivatization are morespecifically useful. Though the utility of these processes for yarns ofany type has not been previously recognized and they have not beenapplied to yarns, useful reaction conditions for carbon nanotubefluorination and plasma-induced derivation are provided, for example, byT. Nakajima, S. Kasamatsu, and Y. Matsuo in European Journal Solid StateInorganic Chemistry 33, 831 (1996); E. T. Mickelson et al. in Chem.Phys. Lett. 296, 188 (1998) and in J. Phys. Chem. B 103, 4318 (1999);and Q. Chen et al. in J. Phys. Chem. B 105, 618 (2001). Other usefulmethods that can be used for chemical derivatization of carbon nanotubesare described by V. N. Khabasheshu et al. in Accounts of ChemicalResearch 35, 1087-1095 (2002); by Y.-P. Sun et al. in Accounts ofChemical Research 35, 1096-1104 (2002); and by S. Niyogi et al. inAccounts of Chemical Research 35, 1087-1095 (2002). Since many of thesemethods decrease the length of single walled nanotubes, there arebenefits of applying these methods to double walled and multiwalledcarbon nanotubes.

For example, the nanofibers in the nanofiber forests used for draw-twistspinning nanofiber yarns can be optionally coated with a hydrophobicmaterial, like poly(tetrafluoroethylene). One method for such coating onnanofibers is by the decomposition of hexafluoropropylene oxide at about500° C. on heated filaments (by hot filament CVD) to produce CF₂radicals, which polymerize to produce poly(tetrafluoroethylene) on thesurface of individual nanofibers (see K. K. S. Lau et al. in NanoLetters 3, 1701 (2003)). Related hot-filament CVD methods can be used toprovide coatings of other polymers, like organosilicones andfluorosilicones. The result of draw-twist spinning of these hydrophobicnanofibers from nanofiber forests is a super hydrophobic twistednanofiber yarn that is useful for water repellent textiles and textilesfor chemical protection clothing. Since the insulatingpoly(tetrafluoroethylene) coats the surfaces of the individualnanofibers (and thereby interrupts inter-fiber electronic transport),such coating using an electrically insulating polymer is useful formaking poorly conducting twisted yarns from originally electricallyconducting nanofibers.

Application of this and related coating methods to fibers that havealready been draw-twist spun enables the retention of nanofiberelectrical conductivity, since the inter-fiber contacts are made duringthe draw-twist spinning and the coating process can be accomplishedwithout interruption of these contacts. Application of tensile stress tothe draw-twist spun yarns can optionally and preferably be used duringthe coating of draw-twist spun fibers with insulators (including solidelectrolytes), so as to minimize any decrease of the yarn's electricalconductivity caused by nanofiber coating with an insulator. A benefit ofsuch coating of nanofibers in draw-twist yarns with an electricalinsulator after the draw-twist process is that the yarn becomes aninsulator-coated wire, which has high electrical conductivity in theyarn direction and is insulating in the lateral direction.

Various useful ways to chemically and non-chemically functionalizenanofibers for various applications have been described in theliterature and these methods can be applied for the twisted nanofiberyarns of the present invention embodiments (see Y. Li, et al., J.Materials Chemistry 14, 527-541 (2004)). The application of thesemethods and like methods to the pre-primary states for the spinningprocess, the primary state for the spinning process, spun yarns,spun-twisted yarns, and yarn assemblies (such as in textiles).

Such insulator coated twisted yarns comprised of highly conductingnanofibers, like carbon nanotubes, are especially useful for diverseapplications, such as wires in electronic textiles (which can be usedfor comfort control in clothing, via providing the possibility ofelectrically heating clothing articles), and insulated wires fortransformers, magnets, and solenoids.

Since the nanofiber yarns of invention embodiments can be knottedwithout undergoing a reduction in strength, unknots that are slip knotscan increase the yarn toughness as measured on a gravimetric basis. Slipknots are unknots that pull out when you pull an end.

The insertion of either individual slip knots or arrays of slip knotsprovide optionally preferred ways to increase gravimetric toughness ofnanofiber yarns. Also, The insertion of either individual slip knots orarrays of slip knots provide optionally preferred ways to change inuseful ways the stress-strain curve of a nanofiber yarn. Once all of theslip knots in a nanofiber array have been pulled out (therebydissipating mechanical energy and contributing to yarn toughness), thestress-strain curve of the originally knotted yarn will approach thatfor the unknotted nanofiber yarn.

8. Composite Formation Using Nanofiber Yarns, Sheets, and Ribbons andComposite and Non-Composite Applications

The nanofibers for nanotube spinning can be optionally coated withvarious inorganic and organic materials either prior to or after thetwisting process. The purpose of this coating can be to provide afriction aid for enhancing the insertion of twist, for conferringspecial functionality to the twisted yarn, or for a combination of thesegoals. These nanofiber coating agents can optionally fill an arbitrarilylarge fraction of the volume of the yarn. However, if the filling factoris high and the materials used for filling have mechanical propertiesthat interfere with the twisting process, high filling is preferablyachieved after the initial insertion of twist.

(a) Nanofiber Yarn/Electrolyte Composites

Since the twisted yarns can be useful for electrochemical applicationsthat utilize the extremely high surface area of nanofibers, a class ofpreferred invention embodiments provide steps in which the twistednanotube yarns are infiltrated with solid or gel electrolytes. Examplesof such applications are as electromechanical artificial muscle yarns,electrochromic yarns, yarn supercapacitors, and yarn batteries.Solid-state electrolytes can also be used advantageously, since suchelectrolytes enable all-solid-state yarn-based electrochemical devices.

Optional and more preferred organic-based solid-state electrolytes arepolyacrylonitrile-based solid polymer electrolytes (with salts such aspotassium, lithium, magnesium, or copper perchlorate, LiAsF₆, andLiN(CF₃SO₂)₂) and ionic liquids in polymer matrices (which can provide awide redox stability range and high cycle life for electrochemicalprocesses). Optional and preferred gel or elastomeric solid electrolytesinclude lithium salt-containing copolymers of polyethylene oxide(because of high redox stability windows, high electricalconductivities, and achievable elastomeric properties), electrolytesbased on the random copolymer poly(epichloridrin-co-ethylene oxide),phosphoric acid containing nylons (such as nylon 6,10 or nylon 6), andhydrated poly(vinyl alcohol)/H₃PO₄. Other optional and preferred gelelectrolytes include polyethylene oxide and polyacrylonitrile-basedelectrolytes with lithium salts (like LiClO₄) and ethylene and propylenecarbonate plasticizers. The so-called “polymer in salt” elastomers (S.S. Zhang and C. A. Angell, J. Electrochem. Soc. 143, 4047 (1996)] arealso optional and preferred for lithium-ion-based devices, since theyprovide very high lithium ion conductivities, elastomeric properties,and a wide redox stability window (4.5-5.5 V versus Li/Li).

Optionally preferred electrolytes for high temperature deviceapplications include ionic glasses based on lithium ion conductingceramics (superionic glasses), ion exchanged β-alumina (up to 1,000°C.), CaF₂, La₂Mo₂O₉ (above about 580° C.) and ZrO₂/Y₂O₃ (up to 2,000°C.). Other optional and preferred inorganic solid-state electrolytes areAgI, AgBr, and Ag₄RbI₅. Some of the proton-conducting electrolytes thatare useful in invention embodiments as the solid-state electrolyteinclude, among other possibilities, Nafion, S-PEEK-1.6 (a sulfonatedpolyether ether ketone), S-PBI (a sulfonated polybenzimidazole), andphosphoric acid complexes of nylon, polyvinyl alcohol, polyacrylamide,and a polybenzimidazole (such aspoly[2,2′-(m-phenylene)-5,5′-bibenzimidazole]).

(b) Composites and Additives for Enhancing Electrical Conductivities

Additives for enhancing the electrical conductivities of the nanofiberyarns of invention embodiments are especially important. Among thepreferred materials for enhancing electrical conductivity are: (1)elemental metals and metal alloys, (2) electrically conducting organicpolymers, and (3) conducting forms of carbon. These additives can beadded to the nanofiber yarns by various known methods for synthesizingor processing these materials, such as by (a) chemical reaction (such asthe chemically-induced polymerization of aniline or pyrrole to make,respectively conducting polyaniline or polypyrrole, the electrode-lessplating of metals, and the pyrolysis of a polymer like polyacrylonitrileto make carbon), (b) electrochemical methods for conducting yarns (suchas the electrochemical polymerization of aniline or pyrrole to makeconducting polymers and the electroplating of metals), and physicaldeposition methods (such as the vapor deposition of metals, theinfiltration of a soluble conducting polymer or a precursor thereforefrom solution, the infiltration of a colloidal solution of a metal orconducting polymer, or the melt infiltration of a metal). Conductingorganic polymers that are preferred for infiltration into twistednanofiber yarns include substituted and unsubstituted polyanilines,polypyrroles, polythiophenes, polyphenylenes, and polyarylvinylenes. Thesynthetic routes to conducting polymers suitable for the preferredembodiments is well known, and are described, for example, in theHandbook of Conducting Polymers, Second Edition, Eds. T. A. Skotheim etal. (Marcel Dekker, New York, 1998).

Diamond, diamond-like carbon and other insulating forms of carboncontaining sp³ hybridized carbons (possibly mixed with sp² and sphybridized carbons) are usefully employed, since they can both insulateelectrically conducting nanofiber yarns and substantially contribute tothe mechanical properties of the yarn. Infiltration or coating of theelectrically conducting nanofiber yarns with these insulating forms ofcarbon is optionally and preferably by CVD processes or by solid-statereaction of infiltrated precursors using thermal or thermal and pressuretreatments. Typical methods that can be employed for formation of suchforms of carbon on the yarn surface or interior are described in (a) A.E. Ringwood, Australian Patent WO8807409 (1988), (b) Y. S. Ko et al., J.of Materials Research 36, No. 2, 469-475 (2001) and (c) J. Qian et al.,J. Mat. Sci. 17, 2153-2160 (2002).

For carbon nanofibers, palladium and palladium alloy deposition(chemically, electrochemically, or by evaporation or sputtering) isespecially useful for making low resistance ohmic interconnectionbetween nanofibers and between the nanofiber yarns and other materials.The use of this metal for enhancing electrical contacts in nanosizeelectronic devices is described by A. Javey, J. Guo, Q. Wang, M.Lunstrom, and H. J. Dai in Nature 424, 654-657 (2003). Palladium hydrideformation by the absorption of hydrogen can be employed for tuning workfunctions so as to minimize contact resistances to and between carbonnanotubes.

(c) Structural Composites

Polymer additives for the twisted yarns and false-twist spun yarns thatare especially preferred for making yarn composites include polyvinylalcohol; poly(phenylene tetrapthalamide) type resins (examples Kevlar®and Twaron®); poly(p-phenylene benzobisoxazole) (PBO); nylon;poly(ethylene terephalate); poly(p-phenylene benzobisthiozole);polyacrylonitrile; poly(styrene); poly(ether ether ketone); andpoly(vinyl pyrrolidone). Epoxies of such types that are useable forforming graphite-epoxy composites are also preferred for inventionembodiments.

Polymers that are pyrolizable to produce strong or highly conductivecomponents can optionally be pyrolized in the twisted, false-twisted, orliquid-state densified nanofiber yarn. Heat treatment and pyrolysis(such as the heat setting in an oxidative environment and furtherpyrolysis of polyacrylonitrile in an inert environment) is preferablyaccomplished while the twisted nanofiber yarn is under tension. Thisstate of tension is preferably one that results in fiber draw during atleast part of the pyrolysis process. Pitch is also an especiallypreferred yarn additive for stretch-facilitated pyrolysis processes thatresult in carbon matrix/nanotube yarns. The nanofiber yarn containingmaterial that can be pyrolized to make carbon (like pitch orpolyacrylonitrile) is preferably either false-twisted or liquid-statedensified, since high twist can undesirably restrict the ability toobtain fiber draw during pyrolysis.

Because of the importance of draw for strengthening matrix polymers andthe improving the properties of materials that are being pyrolized, andthe observation that twist decreases the draw of nanofiber yarns,false-twist spun yarns are especially useful for optimizing theachievable properties of the yarn composites.

Structural materials used for friction generation, especially frictionmaterials used for land vehicle and aircraft brakes, benefit from theemployment of carbon nanotube yarns and sheets of invention embodiments.These structural composites are optionally and preferably carbon-carboncomposites. These carbon-carbon composites are preferably made by inprocesses that involve either the pyrolysis of an organic material thathas been infiltrated into the an array comprising nanofiber sheets,nanofiber yarns, or an array comprising both nanofiber sheets andnanofiber yarns.

When used for brakes, the nanofiber sheets are preferably orientedroughly parallel to the friction surface, such as the disk surface ofland vehicle or aircraft brakes. Optionally and preferably, thesenanotube sheets are transversed in an at least approximately orthogonaldirection by reinforcing yarns or fibers (optionally and preferablycomprising either graphitic fibers or yarns or nanotube yarns ofinvention embodiments). This stitching process can be done using methodswell known in the art.

The formation of carbon-carbon composites using the nanotube sheets andyarns of the present invention can proceed similarly to the conventionaltechnology for carbon-carbon brakes and can utilize similar additives,like those employed for oxidative protection. Optionally preferredmaterials for pyrolysis to form the matrix component of carbon-carbonbrakes are phenolic resins, polyacrylonitrile, and like materials knownin the art.

Gas phase pyrolysis steps can optionally be accomplished, such as thoseusing natural gas as a gas component. Multiple resin and gasinfiltration steps can be usefully employed in order to decrease voidspace and optimize performance.

(d) Composite and Non-Composite Applications

A variety of applications enabled by the high sheet and ribbon strengthsdescribed in Examples 6 and 27. The inventors provide heredensity-normalized strengths that are already comparable to or higherthan the ˜160 MPa/(g/cm³) strength of the Mylar® and Kapton® films usedfor ultra-light air vehicles and proposed for solar sails for spaceapplications (see D. E. Edwards et al., High Performance Polymers 16,277 (2004)) and those for ultra-high strength steel sheet (˜125MPa/(g/cm³)) and aluminum alloys (˜250 MPa/(g/cm³)).

The high strength for the inventors nanofiber sheets, ribbons, and yarnsindicate that the preferred applications modes include use formembranes, diaphragms, solar sails, tents and other habitablestructures, ultralight air vehicles, micro and macro air vehicles,pneumatically supported fabrics (such as domes, balloons, otherinflatable structures, parachutes, and ropes (such as those useful formarine vehicle mooring and tethering objects in space). As analternative or a complement to using pneumatic support, thenanotube-based sheets and ribbons and nanotube-based textiles can bemechanically tensioned, using for example metal tensioning elements.

The utilized yarns and sheets can optionally be plied. For example,oriented nanofiber sheets of invention embodiments (See Example 28 andFIG. 27) can be optionally laminated together so as to produce a pliedsheet structure in which all sheets do not have the same nanofiberorientation direction. In fact, nanofiber sheets in plied sheetstructures can be plied to produce a plied sheet structure that hasanisotropic strength for in-plane (i.e., in-sheet directions).

The nanofiber yarns or sheets can optionally contain a support anotherfunctional material. For example, an infiltrated or overcoated materialis useful for reducing or eliminating gas permeability for membrane,diaphragm, inflatable structure, and pneumatically supported structures.These nanofiber-based structures that are coated or infiltrated includenanotube sheets, ribbons, and textiles incorporating nanofiber yarns, aswell as other possibilities. Yarn, ribbon, and sheet infiltration withpolymer, metal, and other binding agents and matrix materials are alsoespecially useful for providing strength enhancements.

Since the mechanical gravimetric strength of the carbon nanotube sheetsexceeds that of the Mylar that is being exploited for solar sails, thesecarbon nanotube sheets are especially promising for use as solar sails.While it is difficult to make ordinary polymer films sufficiently thin,the inventors have made nanofiber sheets that are so thin (50 nmthickness) that a four ounce sheet could cover an acre.

Such sheets or much thicker sheets of invention embodiments canoptionally be plied for the purpose of making a solar sail that isstrong in all in-plane directions. The extreme radiation and thermalstability of nanotube would be especially useful for the solar sailapplication, as would be the exceptionally high thermal diffusivity ofthese sheets (which would assist in temperature equilibration betweendifferently thermally exposed sheet areas and sheet sides).

Petal configured solar sails comprising nanofibers sheets, nanofiberyarns, or combinations thereof are optionally preferred due toconvenient deployability. Such petal configured solar sail are named forstructural similarly of solar sail blade arrays to those of the openpetal array of a flower. Teachings for the geometry of such solar sailsare in the literature. One type (comprising metallized Mylar) wasrecently on a missile fired into space, but the launch into space wasunsuccessful.

Optionally preferred configurations for petal configured solar sail arethose comprising a plurality of at least approximately triangular petalsthat join at a triangle apex. The angle of the triangle at the apexwhere the triangles are joined is optionally preferably less than 120°and greater than 15°. Also the number of rectangular petals in the solarsail is preferably at least three.

While the nanotube sheets could be used for solar sail applicationswithout being coated or laminated, it is optionally preferable to makethe nanofiber sheets highly reflecting, which can be achieved bydeposition of a thin metal overcoat or laminating the nanofiber sheetwith a highly reflecting material. Such overcoating with a highlyreflecting material can optionally be accomplished for an infiltratednanotube sheet, such as a polymer infiltrated sheet.

The high strength of the carbon nanotube sheets andcarbon-nanotube-based textiles, as well as the achievable toughness,means that they can be incorporated in tires as a tire-cord fabrics,which can optionally be used for providing a sensor responses indicatingtire pressure and the operating conditions of the tire under useconditions. Methods for incorporating nanofiber sheets, nanofibertextiles, and nanofiber yarns into tires, such as rubber tires, can belike those used to incorporate conventional tire cords.

These mechanical properties can enable other applications, likeincorporating into antiballistic composites (including antiballistictextiles), cut resistant gloves and other clothing, space suits, andprotective clothing for moon or planetary missions. The high thermalconductivity and thermal diffusivity of nanotube yarns, sheets, andfabrics can be useful for the various applications, such as fortemperature regulating clothing (such as for space suits and protectiveclothing for the exploration of moons, planets, and other bodies inouter space).

9. Assemblies of Twisted Nanofiber Yarns, Sheets, and Ribbons with OtherFibers

The twisted, false-twisted, and solution densified nanotube yarns ofinvention embodiments can optionally be combined with non-wovens to makestructures that combine the cost benefits of non-wovens with themechanical properties and electrical properties achievable for thetwisted nanofiber yarns. Various combinations can be usefully employed.For example, the twisted nanotube yarns can be embedded within the bodyof the non-woven (such as a non-woven produced by electrostaticspinning) or the twisted nanofiber yarn can be used to stitch thenon-woven, so as to improve mechanical properties of the assembly. Abenefit of incorporating electrically conducting twisted nanofiber yarnsinto the body of electrically insulating non-wovens is that thesenon-wovens can insulate electrically conducting elements intwisted-nanofiber-based electronic circuitry. The non-woven canoptionally be entangled, using such means as a water jet or transversepenetration with a bed of needles.

An electrically conducting nanofiber sheets of invention embodiments canoptionally serve as either the receiving electrode for electrostaticallyspun yarns or an over laid material on this electrode. In either casethe beneficial result is a product that is a laminate of theelectrostatically spun yarn and a nanofiber sheet of inventionembodiments.

Electrically insulating fibers and yarns can optionally be twisted in aspiral manner about the twisted electrically conducting nanofiber yarns,so as to provide electrical insulation and other desired properties.Also, the twisted nanofiber yarns can optionally be twisted aboutconventional fibers and yarns using equipment that is commonly used formaking topologically analogous structures from conventional yarn andfiber structures. Such methods include core and wrap spinningadaptations to conventional ring spinning frames.

Example 53 demonstrates a twist-based method for making a fibercomposite of two different fibrous materials, one comprisingelectronically conducting carbon nanotubes and the other comprisingelectronically insulating cellulose microfibers. Also, this exampledemonstrates a method that provides either the insulating microfibers orthe conducting carbon nanofibers on the outer surface of the twistedyarn. In addition, this demonstration shows how a carbon nanotube yarncan be covered with an insulating layer. Also, by replacing thecellulose sheet with a similar sheet comprising fusible polymermicrofibers (such as polypropylene or polyethylene-based non-wovenpaper), the method of Example 53 can be used to make polymer/nanotubecomposite yarns that are either twisted or false twisted prior to fusionof the polymer onto the nanofibers in the yarn by thermal or microwaveheating. This demonstration uses the tissue paper/nanotube stackcomposite that has been contoured using the method of Example 45. Threemillimeter width ribbons were cut parallel to the nanotube orientationdirection from the composite stack, and twisted to provide a moderatestrength yarn. Apparently because of the contouring on the oval mandrel(with the nanotube fiber direction in the circumferential direction),the inventors found that (depending upon the direction of twist) eitherthe insulating cellulose microfibers or the electrically conductingcarbon nanofibers would appear on the surface of the twisted yarn.

Woven structures used for diverse applications can include the twistednanofiber yarns as either part of or the entire warp or part of or theentire weft. Insulating yarns or fibers can separate electricallyconducting twisted nanofiber yarns in the warp, the weft, or both—so asto electrically separate the conducting twisted nanofiber yarns. Theseconducting yarns can optionally be separated (to avoid unwantedelectrical contact associated with textile bending) by laminatinginsulating textiles or providing insulating coatings on textile sideswhere the conducting twisted nanofiber yarn is exposed. Alternatively,the conducting twisted nanofiber yarn can have such small diametercompared with the insulating yarns or fibers that separate theseconducting yarns that insulation is provided by the conducting nanofiberyarns being buried in the bulk of the textile. For this purpose ofhindering shorting between uninsulated electrically conducting nanofiberyarn wires, the electrically conducting nanofiber yarn wires areoptionally and preferably configured in a woven textile with insulatingyarns having at least five times the diameter of the electricallyconducting nanofiber yarn wires.

FIG. 40 shows a yarn made in the laboratory that combines nanotubecomponents with wool fibers in a twist-based process. The benefit ofsuch assembly is to combine the attractive characteristics of wool withthose of carbon nanotubes. For example, the carbon nanofiber componentcan provide the electrical conductivity needed for yarn application forelectrical heating, and the wool component can provide the beneficialcharacteristics of wool, such as the ability to absorb sweat.

After initial nanofiber yarn, sheet, or ribbon fabrication bysolid-state methods, additional carbon nanotubes or other nanofibers(called secondary nanofibers) can be optionally incorporated intosolid-state spun nanofiber yarns, ribbons, and sheets by a variety ofuseful processes. One process is by adding catalyst to theseprefabricated materials to enable CVD-based growth on this catalyst.Another method is to add catalyst by the thermal decomposition of ametallo-organic during CVD growth of the secondary nanofibers. Thesemethods are especially useful for solid-state spun carbon nanotubesheets, ribbons, and yarns and it is especially useful for theCVD-grown, secondary nanofibers to be carbon nanotubes.

As a result of such additional addition of CVD grown nanotubes topreformed nanotube sheets, ribbons, and yarns a useful hierarchalstructure can be formed, which usefully includes nanofibers grown fromcatalyst on nanofibers, nanofiber bundles, and larger nanofiberassemblies. Advantages can result from such secondary nanofibers topre-formed yarns, sheets, and ribbons and these include enhancements inthermal and electrical conductivity.

Alternatively, secondary nanofibers can be added to pre-formed nanofibersheets of present invention embodiments by filtration processes of thetype typically used for the formation of nanotube sheets on a filtermembrane. In effect, the nanofiber sheet is the filter membrane,although another conventional filter membrane can also be employed tosupport the preformed nanofiber sheet during filtration processes, andsuch support is especially useful when the preformed nanotube sheet isas thin as 30 nm.

10. Applications of Nanofiber Yarns, Ribbons, and Sheets (a) TextileApplications

The surprising nanotube yarn properties resulting from the practice ofinvention embodiments are especially useful for application of thenanotube yarns as either a minority or majority component intwo-dimensional or three-dimensional textiles, including electronictextiles. The inventors have discovered, surprisingly, that high thermaland electrical conductivities can be obtained in combination with highstrength and high toughness by using processes of this invention. Thesmall yarn diameters achieved (one micron) are over ten times lower thanfor conventional textile yarns and for previously reported continuousfibers or yarns comprising only nanotubes.

Highly conducting twisted nanofiber yarns are useful as antennas thatcan be woven into textiles employed for clothing and used to transmitvoice communications and other data, such as information on the healthstatus, location of the wearer, and her/his body motions, as well asinformation collected by the wearer or by sensor devices in theclothing. The configurations employed by such antennas can beessentially the same as for conventional antennas, except that thenanofiber yarn antennas can be woven or sewn into the clothing textile.

Additionally, electrically conducting twisted nanofiber yarns canusefully be employed to make clothing textiles (and textiles used forsuch applications as tents) into large area acoustic arrays for thedetection and location of noise. The nanofiber yarns can connectmicrophones in a textile, which can be as simple as a poledferroelectric polymer that is located at the cross-point betweennanofiber yarns in a textile. These cross-points are optionally andpreferably between nanofiber yarns that are at least approximatelyorthogonal, such as the warp yarn and the weft yarn of a textile, whichare not necessarily both nanofiber yarns. The poling of theferroelectric coating on a nanofiber yarn can be either before or afternanofiber yarn assembly into a textile. However, the poling direction ofthe ferroelectric is optionally and preferably orthogonal to the yarnlength direction and the polling step is optionally and preferablyaccomplished after the textile has been fabricated. This polingdirection can optionally be either within the plane of the nanofiberyarn or orthogonal to this plane. However, this poling direction isoptionally and preferably orthogonal to the plane of the textile or thelocal plane of the textile if the textile is non-planar.

Replacing metal wires in electronic textiles with nanotube yarns canprovide important new functionalities, like the ability to actuate asartificial muscles and to store energy as a fiber supercapacitor orbattery. Polymer-free MWNT yarns of the present invention provide twicethe strength of nanotube fibers used for artificial muscles (R. H.Baughman, A. A. Zakhidov, and W. A. de Heer, Science 297, 787-792(2002)) and the polymer-intercalated MWNT yarns provide a hundred timeshigher electrical conductivity than for the coagulation-spun SWNT/PVAfibers used to make fiber supercapacitors (A. B. Dalton et al., Nature423, 703 (2003)).

Reflecting the micron or thinner yarn and sheet thicknesses demonstratedin the invention embodiments and the low visibility observed for thesethicknesses, as well as the high electrical conductivities demonstratedfor specific compositions (like carbon nanotubes), the yarns and sheetsof the present invention embodiments can be usefully employed astransparent and low visibility substrate materials for making electricalcontact and interconnections. Resulting transparent conductingelectrodes are important for such applications as liquid crystaldisplays, light emitting displays (both organic and inorganic), solarcells, switchable transparency windows, micro lasers, opticalmodulators, electron field emission devices, electronic switches, andoptical polarizers. Inorganic electrodes, like ITO (indium tin oxide),degrade on bending and are costly to apply or repair. The presentinvention embodiments eliminate these problems.

The surprising mechanical strength, abrasion resistance, and resistanceto any degradation of these properties due to knotting makes the twistedyarns of invention embodiments well suited for fabric keyboard switchesfor electronic textiles. Pressure-activated switches of inventionembodiments comprise (a) a textile containing electrically conducting,twisted nanofiber yarns (like carbon nanotube twisted yarns) thatprovides a first switch contact, (b) a first electrical connection thatis to the first switch contact, (c) a second electrical connection madeto a material that is a second switch contact, (d) an insulatingmaterial that breaks direct or indirect electrical conduction betweenthe first and second switch contact (and,

the first and second electrical connection) unless suitable pressure isapplied to the switch, and (e) means to form electrical conductionbetween the first and second switch contact when suitable pressure isapplied.

In one preferred embodiment for such a keyboard switch, an electricallyconducting twisted nanofiber yarn is woven into a first textile so thata surface region of this textile is electrically conducting. Thistextile with conducting surface region, which serves as a first switchcontact, is separated from a second electrically conducting surface byan insulating spacer sheet (such as an insulating textile, or a suitablyconfigured insulating textile fiber or yarn). This insulating materialextends only part of the possible contact region between the firstswitch contact and the second electrically conducting surface, so thatpressure applied approximately orthogonally to the switch surfaceprovides electrical contact between the first switch contact and thesecond electrically conducting surface. This second electricallyconducting surface can be the second switch contact. Alternatively,pressure-induced sheet or textile deflection can bring this secondelectrically conducting surface in joint electrical contact between thefirst and second switch contact, so as to provide an electrical pathbetween these switch contacts. In this latter case, the second switchcontact can be an electrically conducting region of the same textilethat includes the first switch contact.

One or both of the electrically conducting textiles in the aboveswitches can be replaced with an electronically conducting nanofibersheet or sheet portion that has been fabricated by a solid-state drawprocess. The nanofibers in the nanofiber sheet of sheet portion areoptionally and preferably carbon nanotubes, and these carbon nanotubesheets or sheet portions are optionally and preferably derived from acarbon nanotube forest. This electronically conducting nanofiber sheetor sheet portion can optionally be attached to the surface of anothertextile that is electronically insulating. The benefit of suchattachment is to provide mechanical support for the electronicallyconducting nanofiber sheet, especially when the electronicallyconducting nanofiber sheet is so thin that it provides opticaltransparency. Such optical transparency is especially important forproviding the greatest latitude for formulating textile appearance.

Because of the combination of surprising mechanical and electricalproperties that are especially useful for electronic textileapplications, the twisted nanofiber yarns can replace conventional wiresin these textiles. For example, conducting twisted nanofiber yarns ofthis invention can be used as wires for sensors and for clothing thatcontains liquid crystal displays or light emitting elements (such aslight emitting diodes). These twisted nanofiber yarns, and especiallythe twisted carbon nanotube yarns, can replace the conventional wiresused for the electronic textile applications described in by E. R. Postet al. in IBM Systems Journal 39, 840-860 (2000), and similar methodscan be employed for creating device structures from conventional wiresand from these twisted nanofiber yarns.

The twisted yarns of invention embodiments can be employed to make amicrodenier version of Velcro® that provides either permanent or easilyreversible interconnections between opposite surfaces upon theapplication of pressure that brings these surfaces into intimatecontact. In one invention embodiment, the twisted nanofiber yarnsprovide closed loops in a textile base that interconnect with hooks on amating surface. These hooks on the mating surface can, for example, bearrow-like barbs, around which the nanofiber yarns loop when thenanofiber yarn containing textile is pressed upon the neighboringsurface. Alternatively, the hooks can be cut loops of nanofiber yarnthat are infiltrated by a rigid polymer, such as by infiltration of apolymer from a polymer solution or photopolymerization of an infiltratedpolymer. Benefits of such use of twisted nanofiber yarns for thisapplication are many. Using strong, tough carbon nanofiber yarns thatmate with strong hooks (like lithographically-produced diamond hooks)extraordinarily strong and tough interconnections can be made betweenthe mating surfaces, which provide very high thermal conductivitybetween the mating surfaces (as a consequence of the high thermalconductivities of both the carbon nanotubes and materials like diamond).If both sides of the mating surface are electrical conductors,mechanical connection between the two mating surfaces can provideelectrical connection, which can be used, for example, for makingelectrical connections for electronic textiles. Furthermore, patterns ofclosed loops and hooks (or mechanical equivalents) can be provided onboth of the mating surfaces, so that the mating process helps laterallyalign the two surfaces. Moreover, the extremely small presentlydemonstrated nanofiber yarn diameters implies that this means ofconnecting surfaces (textile or solid) can be applied on the hundredmicron scale for microcircuit applications. For such application, forexample, the yarn loops can be anchored in a solid polymer or in ametal, and the opposing barbs can be lithographically produced in any ofvarious possible materials, like silicon, diamond or diamond-likecarbon, a plastic, or a metal.

(b) Knot-Based Electronics and Other Methods for Forming ElectronicDevices from Twisted and Untwisted Nanofiber Yarns

Application of the twisted nanofiber yarns of this invention aselectronic devices (especially those in electronic textiles) is enabledby (1) the demonstrated mechanical robustness and electricalconductivities and the retention of these conductivities when the yarnis infiltrated, (2) the ability to change electrical properties for yarnsegments by chemical modification or doping, (3) the demonstratedabsence of mechanical property degradation when the twisted nanofiberyarns are knotted, and (4) the variety of metallic, semiconducting, andmetallic nanofibers available for the practice of invention embodiments.

Twisted nanofiber yarns made of superconductors, like nanofibers havingthe approximate composition MoS_(9-x)I_(x) (where x between about 4.5and 6) can be used as superconducting cables and as superconductingwires for magnets. Nanofibers of the Nb₃Sn superconductor, the MgB₂superconductor (which has a superconducting transition temperature ofabout 39 K), and the carbon doped MgB₂ superconductor are especiallypreferred as component nanofibers for twisted nanofiber yarns ofinvention embodiments that superconduct (see Y. Wu et al., AdvancedMaterials 13, 1487 (2001), where the growth of superconducting MgB₂nanowires by the reaction of single crystal B nanowires with the vaporof Mg is described). Benefits of using the methods of inventionembodiments are the high strength and high toughness of the twistednanofiber yarns and the intimate electrical interconnections betweennanofibers in these yarns.

Novel methods of invention embodiments have been above described thatprovide controllably patterned variation in electrical properties alonga yarn length, which can be usefully employed for the fabrication ofelectronic devices based on nanofibers yarns. The inventors heredescribe other novel methods that can be employed for device fabricationusing nanofiber yarns.

The inventors refer to the first category of invention embodiments asknot-based electronics, since knot structures are used for thefabrication of electronic devices. One strategy is knot-basedlithography, which can utilize yarn densification at a knot (and,optionally, differences in densification at different places within aknot), to provide the capability of patterned deposition, reaction, orremoval needed for the fabrication of electrical, fluidic, thermal, ormechanical circuits or circuit elements. These methods of patterneddeposition, reaction, or removal can be applied to singles or foldedyarns and to yarns that have been woven or otherwise assembled into astructure. Also, these methods of obtaining region-selective materialdeposition, reaction, or removal can include, among other usefuloptions, exposing the knotted yarn or yarn assembly to a gas; vapor;plasma; liquid; solution; fluid dispersion; super critical liquid; melt;or conditions resulting in electrochemical deposition, electrochemicalmaterials removal, or electrochemical polymerization.

The simplest embodiment of these concepts can be understood by notingfor the twofold yarn in FIG. 6, and the singles yarn in FIG. 12, thatthe region of the yarn that is tightly knotted has a much higher densitythan unknotted regions of the fiber. As a consequence of this densitydifference, the knotted region of the fiber will be much more difficultto infiltrate with a fluid, vapor, or plasma than is the case forunknotted yarn regions. For example, the selectively infiltrated agentcan be a chemical that is used to transform the electrical properties ofthe infiltrated yarn region, or it can be a resist material that servesto protect the infiltrated region when electrical propertytransformations are accomplished for un-infiltrated yarn regions, suchas by chemical or electrochemical doping or by liquid, gas, orplasma-induced chemical transformations. After this process, the resistmaterial can be optionally removed.

Close inspection of FIG. 12 shows that relative yarn dimensions at yarnlocations removed from the inserted overhand knot (1202), at the knotentrance (1203), and knot exit (1204), and in the body of the knot(1201) provide regional density differences that can be used forselective region infiltration and reaction. The pictured stray nanotubesthat migrate from the knot and other regions of the knot can optionallybe removed chemically (such as by passing the yarn through an openflame). If desired for applications like electron field emission, thedensity of these stray nanotubes can be selectively increased indifferent regions of the yarn by mechanical treatments or chemicaltreatments, including chemical treatments that result in nanofiberrupture. Examples of such mechanical treatments are, for example,abrasion between the twisted nanofiber yarns and a rough surface ororifice and the ultrasonication of a twisted nanofiber yarn (optionallyand preferably while tension is applied to the yarn). Examples of suchchemical processes are treatment in oxidizing acids, plasma oxidation,and oxidation in air during thermal annealing, and surface fluorination(which can be later reversed by thermal annealing).

Knots can be formed by all of the methods currently used in the textileindustry. The type of knot formed depends on the size and nature of theparticular construction required. Overhand knots are possible forcompact applications when the knotted length is not too large and theyarn package is relatively small. In the case large-scale applicationswhen more extended knotting is required, knots can be formed using allof the established technologies currently to form loops such asknitting, braiding, and embroidering, which can then be tightened to thedegree required by the application.

Differences in electrical conductivity for knotted and unknotted fiberregions of electrically conductive yarns (like carbon nanotube yarns)can also be used for lithography. One approach is to apply a voltagepulse or sequence of well-separated voltage pulses in order to causepreferential thermal transformations or evaporation of electronicchemicals in the more resistive fiber region. Knotted regions of theyarn will generally have higher electrical conductivity than unknottedregions, so the unknotted regions will selectively increase temperaturerelative to knotted yarn regions when a voltage pulse is applied. On theother hand, the higher porosity of unknotted regions of the fiber can beused to reverse this effect, since the higher porosity of unknotted yarnregions means that the temperature increase during continuous electricalheating is reduced relative to that for knotted yarn regions.

Knotted twisted nanotube yarns are preferred for selected applicationsof these yarns, some of which are described above. Special types ofknotted twisted nanofiber yarns are also preferred for selectedapplications, such as where independently tied knots (called knotfactors) are assembled so that they partially or completely overlap onthe twisted nanofiber yarn.

The nanofiber yarns of invention embodiments can be optionally patternedto provide semiconducting, metallic, or superconducting regions eitherbefore or after incorporation of these yarns into textiles. Thispatterning can be by any of various means, such as (a) application ofwell known lithographic or soft lithography methods, (b) ink jetprinting, or (c) laser printing methods. These methods are optionallyand preferably multi-step and can combine the various well-known methodsof pattern formation, such as photo-polymerization or electron-beaminduced reactions of polymers; pressure-induced material transfer; andliquid, gas phase, or plasma treatments to deposit, remove, or transformmaterials.

Methods can be employed for using the conducting yarns as interconnectsfor self-assembled functional devices, like electronic chips. Suchmethods can utilize the extremely small diameter electrically conductingyarns that can be produced by the methods of invention embodiments, theability to create woven structures containing patterns of preciselyshaped depressions, and the ability to insulate different lengths ofyarn in a textile with respect to one another. Using shape effects,patterned surface tension variations, or (most desirably) a combinationof these effects and possibly other self-assembly effects, functionaldevices (such as transistor chips) can be self-assembled on a textile bydepositing a fluid containing the chips on the textile. FIG. 16 providesa schematic picture of a textile weave that provides docking sites forfunctional devices (such as substrate-released electronic chips), whichcould be self-assembled onto the textile at these docking sites from aliquid-based dispersion of the particle-like devices. Element 1601 is anelectrically conducting, twisted nanofiber yarn that is insulated fromall pictured like elements. Element 1602 and all like-shaped holes arepossible docking sites for the functional devices.

Related methods have been employed for self-assembling electronic chipson planar and curved substrates, like plastic sheets containing metallines for interconnections (see K. D. Schatz, U.S. Pat. No. 6,780,696;T. D. Credelle et al., U.S. Pat. No. 6,731,353; J. S. Smith et al., U.S.Pat. No. 6,623,579 and U.S. Pat. No. 6,527,964; M. A. Hadley et al.,U.S. Pat. No. 6,590,346; G. W. Gengel, U.S. Pat. No. 6,417,025). Theteachings of this prior art can be used to provide useful variations onthe present invention embodiment where the twisted nanofiber yarns in atextile are used together with other textile components for thefluid-based self-assembly of electronic chips in an electronic textile.The various methods described in this prior art show means forconnecting metal lines, to the self-assembled electronic chips, and itwill be obvious to one skilled in the prior art as to how these andrelated methods can be applied to provide interconnections between thenanofiber yarns and functional devices (like electronic chips andmicrofluidic circuit elements) that are self-assembled on textiles.

(c) Wire Applications

The nanofiber yarns can be used as wires, and wires capable of carryinghigh currents. Carbon nanotube twisted yarns, and especially such yarnscontaining a conductivity enhancement aid are especially useful for thetransport of electrical currents. Advantages obtained for these twistedcarbon nanotube yarns are high current carrying capacity, hightemperature stability, and freedom from electro-migration effects thatcause failure in small diameter copper wires. The low weight and highmechanical strength of these twisted nanofiber yarns can be especiallyuseful for aerospace and space applications where weight is especiallyimportant, and for applications where it is useful to employ wiring toprovide mechanical reinforcement. Other potential applications are, forexample, as power cables and as the windings of magnets, transformers,solenoids, and motors, and for these devices that are incorporated intoa textile.

The electrically-conductive connections between yarns, or between yarnsand other materials, can be made by using conductive gels (such assliver paints), knotting, or mounting.

(d) Electrochemical Device Applications—Supercapacitors, Batteries, FuelCells, Artificial Muscles and Electrochromic Articles

Because of the high achievable porosity of the twisted nanofibers andthe high electrical conductivity demonstrated herein for particulartwisted yarns (such as twisted carbon nanotube yarns, both before andafter infiltration with electrolyte) these twisted yarns are useful aselectrodes for yarn-based electrochemical devices that use eitherelectrochemical double-layer charge injection, faradaic chargeinjection, or a combination thereof. These devices could utilize eitherelectrolytes that are liquid state, solid-state, or a combinationthereof (see above discussion of electrolytes).

Examples of twisted yarn electrochemical devices of this inventioninclude supercapacitors, which have giant capacitances in comparisonwith those of ordinary dielectric based capacitors, andelectromechanical actuators that could be used as artificial muscles forrobots. Like ordinary capacitors, carbon nanotube supercapacitors (A. B.Dalton et al., Nature 423, 703 (2003)) and electromechanical actuators(R. H. Baughman et al., Science 284, 1340 (1999)) comprise at least twoelectrodes separated by an electronically insulating material, which isionically conducting in electrochemical devices. The capacitance for anordinary planar sheet capacitor inversely depends on the inter-electrodeseparation. In contrast, the capacitance for an electrochemical devicedepends on the separation between the charge on the electrode and thecountercharge in the electrolyte. Because this separation is about ananometer for nanotubes in electrodes, as compared with the micrometeror larger separations in ordinary dielectric capacitors, very largecapacitances result from the high nanotube surface area accessible tothe electrolyte.

These capacitances (typically between 15 and 200 F/g, depending on thesurface area of the nanotube array) result in large amounts of chargeinjection when only a few volts are applied. This charge injection isused for energy storage in nanotube supercapacitors and to provideelectrode expansions and contractions that can do mechanical work inelectromechanical actuators. Supercapacitors with carbon nanotubeelectrodes can be used for applications that require much higher powercapabilities than batteries and much higher storage capacities thanordinary capacitors, such as hybrid electric vehicles that can providerapid acceleration and store braking energy electrically.

The construction of a twisted yarn electrochemical device that can beused as a supercapacitor, an artificial muscle, or a battery is providedin Example 18. These twisted yarns can be incorporated as threads intextiles. While incorporation of coagulation spun nanofibers assupercapacitors in textiles has been previously shown (see A. B. Daltonet al., Nature 423, 703 (2003)), these threads have neither the degreeof twist or the high ratio of nanofiber length to fiber circumferenceneeded for enhancing the mechanical properties by insertion of twist.Also, these prior-art fibers have about an order of magnitude lowerelectrical conductivity than the highly twisted carbon nanotube yarns ofinvention embodiments.

Various methods can be employed for effectively employing the nanofiberyarns of invention embodiments in thermochromic devices, including thosethat are woven or otherwise arrayed in electronic textiles. One methodis to use the twisted nanofiber yarns as heating elements to cause thecolor change of a thermochromic material, such as a liquid crystal, thatis overcoated or otherwise incorporated into the nanofiber yarn.

Another method is to utilize electrochemically-induced color changes ofan electrically conducting nanofiber-yarn electrode that is infiltratedwith or coated with an electrolyte. For this method, thecounter-electrode can be another twisted nanofiber that contacts thesame electrolyte as for the working electrode, but other usefulpossibilities exist. For example, the counter-electrode can be anelectrically conducting coating on the textile that is separated fromthe twisted nanofiber electrode by the ionically conducting electrolytethat is required to both avoid inter-electrode shorting and to providean ion path. The electrochemically-induced chromatic change of thenanofiber yarn in either the infrared, visible, or ultraviolet regionscan involve either faradaic processes or non-faradaic charge injection,or any combination thereof. Electrically conducting twisted nanofiberyarns overcoated with a conducting organic polymer (or twisted nanofiberyarns comprising conducting polymer nanofibers) are optionally preferredfor color change applications, and especially as yarn electrodes thatprovide color changes in electronic textiles. Twisted carbon nanofiberyarns are optionally especially preferred as electrodes that changecolor when electrochemically charged either faradaically ornon-faradaically. These chromatic changes occur for the carbon nanotubefibers in the useful region in the infrared where the atmosphere istransparent.

Using these chromic materials, electronic textiles that providepixilated chromatic changes can be obtained. Methods for electronicallyaddressing individual pixels are widely used for liquid crystal displaysare well known and the same methods can be used here. For example,applying a suitable potential between the ends of orthogonal yarns in atextile will selectively heat a thermochromic material separating theseyarns and that has a much lower electrical conductivity than the yarns.

Conducting twisted nanofiber yarns are especially useful as fuel cellelectrodes that are filled with electrolyte and contain catalyst.Because of their strength, toughness, high electrical and thermalconductivities, and porosity, the twisted carbon nanotube yarns areincluded among preferred compositions for the fuel cell application. Thefuel cell electrode can comprise a singles or folded yarn (together witha penetrating electrolyte and a catalyst such as Pt), or it can comprisean array of twisted yarns, especially including those that have beenwoven (or otherwise configured) into a textile.

FIG. 104 schematically illustrates a fuel cell of invention embodimentsthat can be configured in the form of a yarn-size and yarn-shape devicethat can be woven into a textile, or as a much larger diameter device. Acarbon nanotube yarn hydrogen electrode (10402), which can be anassembly of nanotube yarns, is coated with solid electrolyte (10403) andin intimate contact with this electrolyte and the electrolyte is also inintimate contact with a surrounding braided nanotube yarn oxygenelectrode (10401). Both 10401 and 10402 contain sufficient nanoporousvoid space that air (or oxygen) makes contact to 10401 and hydrogen canbe transported through 10402. A catalyst, typically of Pt or a Pt alloy,is preferably in the contact region between these gases and theserespective electrodes. The hydrogen fuel is transported through theporous regions of nanotube yarn electrode 10402 and oxygen (or air) isdelivered to the oxygen electrode 10401. The hydrogen fuel canoptionally be replaced by an alternative fuel, like hydrazine ormethanol.

It is important to provide reliable access of fuel to electrode 10402.Several approaches can be used, internal fuel storage, external fuelstorage, and intermittent internal and external fuel storage. Theexternal storage approach is to have the fuel source (such as hydrogen)separate from the yarn fuel cell. The internal storage approach is tostore the hydrogen producing fuel in the hydrogen electrode of the fuelcell yarn. In either case, fuel storage and access of the fuel to theelectrode can be in and through hollow regions of a yarn electrode 10402(such as the central region of a hollow braided yarn) or in and throughthe porosity of the MWNT yarns, which readily wicks liquids.

Such yarn fuel cells are particularly promising for application on microair vehicles, as small as a dragon fly. Imagine a 30 micron or smallerMWNT yarn that is braided (like a shoe lace) to make a hollow braidedtube that is the hydrogen electrode. This electrode can be overcoatedwith a H⁺ transporting electrolyte layer on the surface of the braidedstructure, and wrapped on a sacrificial mandrel to make the first layerof the shell or wing of a micro air vehicle. The oxygen electrode yarn(which need not have a hollow construction) could then be wound over thetop of the hydrogen electrode, imbibed with additional electrolyte, andchemically treated so as to remove electrolyte only from the yarn sidethat is on the surface of the vehicle (so that triple point contactinvolving air is insured).

As an alternative to this type of construction, a fuel cell yarnstructure 30 microns or smaller in diameter could be made by imbibing anelectrolyte into the outer surface of a 10 micron diameter MWNT yarn,twisting this hydrogen electrode MWNT yarn with a second yarn while theelectrolyte is still sufficiently wet to provide partial infiltration onthe electrolyte contacting side with the electrolyte. Wicking hydrazineinto the hydrogen electrode yarn then makes the fuel cell operational(using the wicked hydrazine as one fuel component and air as the secondfuel component). A catalyst like Pt or a Pt alloy is naturally usefulfor both fuel cell electrodes and, using methods of the prior art, suchcatalyst can be easily deposited in a region selective way in nanotubeassemblies.

Twisted nanofiber yarns wrapped on a spindle are especially preferredfor many of the above applications. This spindle can be one that is partof the final device, or it can be one that is used for arraying thetwisted nanofiber yarn and then removed in following fabrication steps.

(e) Sensors

Twisted nanotube yarns of invention embodiments have special utility aschemical and mechanical sensors that can be optionally knitted or woveninto textiles. These nanotube yarns can also be incorporated intocomposite structures to sense mechanical deformation of these structuresand the occurrence of damage-causing events (before they result incatastrophic structure failure). The mechanical sensor application canuse the change in yarn electrical conductivity that occurs when the yarnis deformed, or the interruption in electronic transport that occurswhen the yarn is broken. For example, twisted nanotube yarns in asoldier's uniform could provide an electrically transmissible signalindicating that a soldier has been wounded in a particular location,thereby enabling effective triage. Also, the toughness of the nanotubeyarns could provide some degree of protection against injury.

Chemical sensor applications of the twisted nanofibers yarns can utilizethe sensitivity of electronic transport and thermal power to theabsorption of chemical on the nanofibers, as well as the reaction ofchemicals or biological agents with derivatized or non-derivatizedsurfaces. This sensitivity of carbon nanotube electrical conductivityand thermal power is well known (see P. G. Collins, K. Bradley, M.Ishigami, and A. Zettl, Science 287, 1801 (2000) and J. Kong et al.,Science 287, 622 (2000)). The benefit that the nanofiber yarns provideis retention of the high surface area of the nanofibers in amechanically robust structure that can be incorporated in a variety ofconfigurations, including as chemical sensors in electronic textiles.

Changes in the electrochemical capacitance of nanofibers in a nanofiberyarn that comprises an electrolyte can also be usefully employed forproviding the response of a nanofiber-yarn-based chemical sensor(including a biochemical sensor). In this invention embodiment twonanofiber yarns separated by electrolyte can optionally be used for thedevice configuration.

Examples 32 and 90 show methods for fabrication of elastomericallydeformable carbon nanotube sheets. These elastically deformable nanotubesheets can be used as stress and strain sensors, wherein the sensorresponse is provides by a change in resistance of the nanotube sheet inresponse to an applied stress or strain. While the stress-strainsensitivity of resistance of the nanofiber sheets is low, which ishighly desirable for most applications, the size or stretch-inducedresistance changes are large enough to be usefully measurable. Moreover,the size of stress-induced resistance changes can be enhanced by coatingeither the nanotube forest (for forest-based spinning) with a materialthat provides a large strain dependence of resistivity (like a suitableselected conducting polymer), by coating or infiltrating the nanofibersheet with such material, or by using a device based on two nanotubesheets, separated by a material having a high strain dependence ofresistivity. In this latter case, the sensor response is determined by achange in inter-sheet resistivity (or a combination of inter-sheet andintra-sheet resistivity).

Piezoelectric and ferroelectric based stress, strain, or pyroelectricsensors can exploit the electronically conducting nanofiber sheets ofinvention embodiments as electrodes on one or both sides of apiezoelectric or ferroelectric sheet.

For pyroelectric sensors used for the detection of radiation (such aslight or infrared radiation) the nanofiber sheet or sheet stack used forone or both electrodes can be chosen to be sufficiently thick to convertradiation to heat. Multiple nanotube sheets can be stacked to obtainappropriate electrode thickness for radiation absorption. These sheetscan either be stacked to eliminate the effect of sheet anisotropy (forexample by orthogonally aligning neighboring sheets) or these sheets canbe aligned so that the orientation directions of the sheets areparallel. In the latter case, the pyroelectric device becomes sensitiveto the polarization of neighboring sheets. Using the methods describedin Section 10(k), pixel-sensitive responses can be obtained for apyroelectric radiation detector.

Section 10(k) further elaborates of the use of carbon nanotube sheetsand woven as sensors, and exploits the electrical anisotropy of thesesheets for obtaining sensor responses that can be monitored for a pixelarray.

Numerous publications on the application of nanotubes and othernanofibers as sensors have been published (see, for example, J. Li etal. Nano Letters 3, 929 (2003) and J. Kong et al., Science 287, 622(2000)) and the teachings of this prior art will facilitate applicationof the present invention embodiments.

As alternative to using the nanotube sheets of invention embodiments asone or more electrodes for a piezoelectric sensor, the same type ofdevice can be operated in reverse direction as a piezoelectric orferroelectric loudspeaker. One benefit is the transparency obtainablefor these strong nanotube sheets and the ease at which they can beflexed without loss of electrical conductivity. Using these transparentnanotubes sheets as electrode on both sides of a piezoelectric orferroelectric sheet enables the fabrication of windows and picturescoatings that are transparent loud speakers.

The methods described in Section 10(I) can optionally be used forimbedding the nanotube electrodes in a ferroelectric sheet material.Poling of the ferroelectric can optionally be accomplished after theembedding process.

The nanofiber electrodes for these sensor and loudspeaker applicationsare optionally preferably densified, especially since this densificationincreases nanotube sheet strength.

(f) Incandescent Light Emission Devices

While it is well known that carbon nanotube yarns can be used asincandescent light sources, the nanotube assemblies of the prior art areuntwisted (see K. Jiang et al. in Nature 419, 801 (2002) and in U.S.Patent Application Publication No. US 2004/0051432 A1 (Mar. 18, 2004);P. Li et al. in Applied Physics Letters 82, 1763-1765 (2003); and J. Weiet al. in Applied Physics Letters 84, 4869-4871 (2004)). The benefit ofinserting twist to form nanofiber yarns of the present invention is thatthe spinning process confers mechanical robustness that translates toincreases in the lifetime of the incandescent filament and to the degreeof repeated mechanical shock that the incandescent filament canwithstand without failure.

The absence of significant strength or toughness decrease due toknotting, as well as the low electrical resistance of knots used to tieseparate nanofiber yarns together; can be employed for this and otherdevice applications.

Additionally, the ability of the spun and twisted carbon nanofiber yarnsto undergo knotting and the very small yarn diameters that can be spunby the methods of invention embodiments (ten times smaller than those ofprior art yarns) enables localization of incandescent heating andelectron beam emission either at knots or in regions between knots. Theobtainable localization of incandescent electrical heating at knots canbe usefully employed to provide an incandescent light source havingmicron and smaller diameter, corresponding to the knot dimension.Various methods can be employed for selectively increasing theelectrical resistance at knots relative to unknotted regions of the yarn(such as selective chemical reaction at the knot).

Also the mechanical durability and resistance to strength degradationdue to knotting can be employed in the fabrication of nets and textilesthat can serve as incandescent heating structures.

Both multiwalled and single walled carbon nanotubes are especiallyuseful for use as incandescent light sources. Unless the goal is tomaximize the ratio or infrared light emission to visible light andultraviolet light emission or to maximize lifetime, the nanofiberincandescent lamps are optionally preferably operated at above 1500° C.in order to enhance electrical to photonic light emission efficiency.Optionally and more preferably when visible light emission is the goalthe nanotube-based incandescent light element can be operated at atemperature of above 2000° C. Optionally and more preferably, whenlifetime maximization is not necessary the nanotube-based or othernanofiber-based incandescent light element is optionally preferablyoperated at a temperature of above 3000° C.

These nanofiber-based incandescent light sources are preferably ones inwhich either an inert gas (such as argon, krypton, or xenon) or a vacuumsurround the nanofiber incandescent element.

Secondary nanofibers can be added to original nanofibers for theformation of incandescent light elements. For example, catalystparticles such as metal or metal alloy particles can be incorporated inthe volume (or on the surface) of an electrically conducting nanotubeyarn, ribbon, or sheet, either before, during, or after a draw processto make a nanotube yarn, ribbon, or sheets for incandescent light orother applications. Well known CVD methods can be used to grow nanotubesfrom these catalyst particles (see references below) that extend fromthe original nanotubes, so as to provide nanofiber comprising elementsfor incandescent lights (as well as field-emitting nanofibers yarns,sheets, or ribbons). These nanofibers for the original yarn, sheet, orribbon are optionally and preferably carbon nanotubes.

Various variations can be made on these processes for adding secondarynanofibers to a primary nanofiber structure. These include, amongothers, (a) the growth of the nanofibers on a pre-primary or primarynanofiber array before formation of yarn, sheet, or ribbon, (b) theaddition of the secondary nanofibers by solution-based infiltration ofpre-formed secondary nanofibers, and (c) delivery of the catalyst grownfrom the gas phase for the growth of the secondary nanofibers.

Growth of nanofibers within or on the nanofiber yarns of inventionembodiments has wider application than solely for the purpose offabricating incandescent light elements or electron field emissionelement. These methods can be used for such purposes as (a) mechanicalreinforcement of the nanofiber yarn, (b) enhancing the electrical orthermal conductivity of the yarn, and (c) providing nanofibers thatextend from the yarn to thereby electrically, thermally, or mechanicallyinterconnect the yarn with surrounding elements, such as other nanofiberyarns, other fibers, or a matrix material. These processes typicallyinvolve the steps of (1) incorporating active catalyst particles in ananotube yarn or precursor nanofiber arrays, and (2) synthesizingnanofibers in a nanofiber yarn or on the surface of a nanofiber yarn byreaction catalyzed by catalyst particles introduced before, during, orafter a twist process is applied for the yarn. If this nanofiber yarn isincorporated into a textile, this particle-catalyzed growth ofnanofibers within or on the nanofiber yarn can be carried out eitherbefore or after the yarn is incorporated into a textile or other yarnarray. This synthesis of nanofibers using catalyst particles can be byCVD, liquid phase synthesis, or other known means. Useful catalysts andcarbon nanotube growth methods that can be employed are described, forinstance, in R. G. Ding et al., Journal of Nanoscience andNanotechnology 1, 7 (2001); J. Liu et al., MRS Bulletin 29, 244 (2004);and S. M. Bachilo et al. Journal of the American Chemical Society 125,11186 (2003). Catalysts and growth methods for other nanofibers aredescribed, for instance, in Y. Wu et al., Advanced Materials 13, 1487(2001); R. Tenne, Angewandte Chemie Int. Ed. 42, 5124-5132 (2003); andX. Duan and C. M. Lieber, Advanced Materials 12, 298-302 (2000), wheresemiconductor nanofibers having high purity are made usinglaser-assisted catalytic growth.

Example 29 shows a stable, planar source of polarized ultraviolet,visible and infrared incandescent light (FIGS. 31, A and B) for sensors,infrared beacons, infrared imaging, and reference signals for devicecalibration. This nanotube sheet incandescent light has the advantage ofproviding highly polarized radiation (as shown in Example 29, the degreeof polarization of emitted radiation increases from 0.71 at 500 nm to0.74 at 780 nm (FIG. 32), which is substantially higher than the degreeof polarization (0.33 for 500-900 nm) previously reported for a 600 μmlong MWNT bundle with ˜80 μm emitting length.

Cost and efficiency benefits result from decreasing or eliminating theneed for a polarizer, and the MWNT sheet provides spatially uniformemission over a broad spectral range that is otherwise hard to achieve.The low heat capacity of these very low mass incandescent emitters meansthat they can turn on and off within the observed 0.1 ms or less invacuum, and provide current modulated light output on a shorter timescale.

The polarized nature of the emitted light from nanotube sheets (andother oriented electrically conducting oriented nanofiber sheets) can beused for reducing glare. For this purpose the orientation of thenanofibers in the sheet are preferably oriented in at least anapproximately vertical direction.

The combination of electrical conductivity and transparency for nanotubesheets and ribbons is also usefully employed for incandescent elementsthat should be transparent, such as electrically heated furnaces andovens (where the benefit is to provide high visibility for the heatedcontents of the furnace or oven). Also, the transparent nanotube sheetsof invention embodiments will be nearly invisible until electricallyheated for the purpose of generating incandescent radiation.

The nanotube sheet incandescent elements of the present inventionembodiments can be optionally plied to increase filament strength. Thisplying can be optionally by crossing the orientation direction ofsheets, so that in-plane mechanical anisotropy is largely eliminated.Such plying can be used to convert a nanotube sheet incandescent lightto one that emits largely unpolarized light.

(g) Protective and Temperature Regulating Clothing Applications

The surprisingly high yarn toughness demonstrated for the nanofiberyarns, as well as the extremely small demonstrated yarn diameters,indicates the utility of the twisted nanofiber yarns as textiles forprotective clothing. Very tight yarn weaves, like those used for sailcloth, are especially useful for stab and puncture resistance. The hightemperature stability of the draw-twist carbon nanotube yarns areespecially useful for making hard armor that involves incorporating thenanofiber yarns in a matrix, like a ceramic, that is processed at hightemperatures. While graphite fibers have as high a thermal stability,the toughness of the twisted carbon nanofiber yarns of inventionembodiments (20 J/g and above) is higher than that for graphite fibers(about 15 J/g).

Electrically conducting nanofiber yarns of invention embodiments can beincorporated into textiles to provide the ability to heat the textile.Also, the electrically conducting nanofiber sheets of inventionembodiments can be laminated between layers of ordinary textiles toprovide the ability to heat the textile by passing electrical currentthrough the nanofiber sheet. These nanofiber yarns and sheets absorb inthe ultraviolet range, thereby providing protection to the effects ofsolar radiation exposure for those wearing other wiseultraviolet-transmissive clothing.

Other means for moderating temperature changes for textiles is to usethe porosity of nanofiber yarns and nanofiber sheets to store phasechange materials, whose heat of fusion absorbs thermal energy whentemperature is becoming too high and releases thermal energy when thetextile temperature is too low. For use in textiles for clothing, thetemperature range of heat absorption and release is preferably chosen tobe within the comfort range of the wearer.

(h) Application as Absorptive Materials for Gases, Liquids, and Solids

The porosity, high surface area, small yarn diameters, and highmechanical strength of the nanofiber yarns, sheets, or ribbons ofinvention embodiments make them ideal materials for concentrating,separating, storing, or releasing gas and liquid components. They arealso useful for concentrating, separating, and storing solids, such asparticulate solids and solids that can be collected in solid form from avapor or liquid and optionally subsequently either released or partiallyreleased in vapor, liquid, liquid component, reaction product, solidforms, and combinations thereof. Such solids include biological agentssuch as bacteria and viruses, which can optionally be at least partiallypyrolized or otherwise modified during a release process.

Carbon nanofibers are a particular type of nanofiber that is useful forthese applications. The gravimetric surface area of nanofibers for thesecollection, separation, storage, or release purposes is optionallypreferably above 10 m²/g and more optionally preferably above 100 m²/g.This surface area can be optionally measured using the well known BETmethod,

The above nanofiber assemblies are especially important forconcentration of analytes present in gases and liquids, and theirsubsequent release by heating, other means, or combinations thereof.Nanofibers assembled into yarns, sheets, ribbons, and combinations ofthese assembly methods are optionally preferable for applications formaterials adsorption or absorption, materials separation, and materialsrelease.

The high electrical conductivity for twisted yarns made of suchmaterials as carbon nanofibers facilitate their use as a material forgas component separation, concentration, and analysis. In a typicalprocess for the use of these conducting materials for this purpose, acarbon nanotube yarn, sheet, or ribbon is exposed to an analyte for atime enabling either separation or concentration by absorption on thehigh surface area of the nanotubes. This absorbed material can then bereleased by heating the nanofiber yarn electrically, by radiofrequencyor microwave absorption, or by the absorption of radiation atultraviolet, visible, or infrared wavelengths.

Materials collected on a nanofiber yarn, ribbon, or sheet (or acomponent derived there from) can be optionally subsequently analyzedwhile on these articles using spectroscopic or other means, or releasedas a gas from these articles and optionally analyzed by analyzing thegas. This gas can optionally be accomplished using such means as massspectroscopy and gas chromatography. Materials collected on thenanofiber yarn, ribbon, or sheet (or a component derived there from) canoptionally be released into a liquid media, and subsequently separatedor analyzed using conventional liquid-based separation or analysismeans.

The collection, separation, or release of solids, liquids, or gases fromthe nanofiber comprising yarns, ribbons, or sheets (or a componentderived there from) can optionally be electrically enabled, such as byheating or capacitive charging in a capacitive device means thatincludes at least two electrodes. For instance, the capacitive chargingin a device means can be by applying a potential between twoelectronically separated electrodes, wherein at least one of theseelectrodes comprises a nanofiber sheet, ribbon, or yarn (or a componentderived from the same). Electrochemical charging can usefully beemployed by incorporating an electrolyte into the inter-electroderegion.

The nanofibers of these invention embodiments can optionally be reacted,surface derivatized, or surface coated to optimize the materials uptakeand materials release processes of this section. The coating canoptionally involve biological agents, such as proteins, antibodies, DNA,or aptamers.

Also, the uptake and release of materials by nanofibers in theseembodiments can be optionally measured by measuring weigh uptake, suchas by using a surface acoustic wave device or a scale, by measuringelectrical conductivity, or by measuring thermopower.

The materials used for the embodiments of this section are optionallypreferably nanofiber sheets, ribbons, and yarns fabricated by asolid-phase process. The nanofibers are optionally preferably carbonnanotubes.

(i) Applications as Channels of Microfluidic Circuits

The porosity of twisted nanofiber yarns can be usefully used as channelsof microfluidic circuits. These microfluidic circuits can be employed,for example, to make a centimeter scale or smaller “fiber laboratory”for chemical and biochemical analysis or, more exclusively, for chemicalsynthesis.

The novel aspect is to use the wicking capability of twisted nanofiberyarns for the transport of chemicals for subsequent possible mixing andchemical reaction, separation (optionally along yarn lengths), andchemical analysis.

FIG. 13 shows a junction that could be used as a junction formicrofluidic applications. This junction comprises an overhand knot(1305) tied in one twofold MWNT yarn (having fluid entrance along 1301and fluid exit along 1302), so that the knot includes a second twofoldMWNT yarn (with fluid entrance along 1303 and fluid exit along 1304).The nanofiber yarns in each of the twofold yarns can optionally differand can optionally be different for the two twofold yarns. Dependingupon the inserted tightness of knot 1305, optionally different fluidsentering along 1301 and 1303 will mix to produce optionally differentfluid mixtures exiting along 1302 and 1304. With increasing tightness ofthe knot, the fluid components entering at 1301 will increasingly exitalong 1304 and the fluid components entering along 1303 willincreasingly exit along 1302. Fluid transport along these yarns canoptionally be varied by applying alternating or steady potentialsbetween the entrance and exit portions of the twofold yarns and betweencomponent single yarns in each of the twofold yarns if the componentsingles yarns in each twofold yarns are electrically insulated withrespect to each other and if these yarns are electrically conducting.

These and many other types of microfluidic circuits based on yarns canbe optionally arrayed on a curved or linear surface to make the finaldevice configuration. As another preferred configuration, thesemicrofluidic yarns can be optionally woven, sewn, embroidered, orotherwise configured in a textile. To well define the microfluidiccircuit, a fraction of the other yarns or fibers can be madesubstantially non-interacting with the microfluidic circuit, such as byappropriately choosing (or modifying) theirhydrophobicity/hydrophilicity and/or porosity. These yarn-basedmicrofluidic circuits can optionally comprise more than one textilelayer, and microfluidic yarns in one textile layer can optionallytraverse between textile layers. Also, the micro-fluidic nanofiber yarnstructure (such as a yarn in a composite) can optionally providemechanical reinforcement of a composite structure. Additionally, andoptionally, said nanofiber yarn can contain a material, such as thoseknown in the art, which mechanically reinforces a structure at the onsetof yarn failure.

Such microfluidic circuits can optionally be used for various purposes,as for textiles in clothing that analyze biological products for healthmonitoring purposes. Also, microfluidic mixtures (like illustrated inFIG. 13 can be used for the mixture of fuel and oxidant for miniaturefuel cells and combustion engines, which could be used for miniaturerobots or micro-air vehicles.

(j) Tissue Scaffold and Other Biological Applications

The spun yarns and sheets of invention embodiments can also be used asscaffolds for the growth of tissue in either culture media or inorganisms, including humans. Examples of possible application includeuse of the nanotube yarns as scaffolds for neuron growth after brain orspinal cord injury. Recent work has shown (see H. Hu, Y. Ni, V. Montana,R. C. Haddon, V. Parpura, Nano Letters 4, 507 (2004); J. L. McKenzie etal., Biomaterials 25, 1309 (2004); and M P. Mattson et al., J. ofMolecular Neuroscience 14, 175 (2000)) that functioning neurons readilygrow from carbon nanotubes, and that carbon fibers having diameters ofabout 100 nm or less retard scar growth and facilitate desired cellgrowth. For the purposes of modifying biocompatibility, the spunnanotubes in the spun yarns and sheets can optionally be chemicallyderivatized or non-chemically derivatized, such as by wrapping with DNA,polypeptides, aptamers, other polymers, or with specific growth factorslike 4-hydroxynonenal. The carbon nanotube yarns and sheets of inventionembodiments can be produced free of any additives (but selectedadditives can be incorporated and the nanotube yarns can be derivatizedif desired), are highly electrically conducting, and are very strong.Also advantageous for medical applications, and unlike other highperformance fibers/yarns (like Kevlar® and Spectra® fibers used forantiballistic vests), these tough yarns are highly resistant to strengthdegradation due to either knotting or abrasion and have a controllabledegree of substantial elasticity. These yarns can be woven into eithertwo- or three-dimensional textiles that could serve as frameworks forthe growth of blood vessels and nerves. The textiles could havevirtually any desired topology: the inventors have made tubularstructures from nanotube yarns and from wrapped spun ribbons that havethe diameter of moderately small blood vessels (see Example 11). Thenanofiber yarns of invention embodiments can be used as electricalconnections to neurons in the brain, ear (for the detection of sound),or eye (for the detection of light), where functioning neurons are grownon the nanofiber yarn to make electrical connection to existing neurons.Neuron growth on the tip of nanofiber yarns having a diameter of lessthan 10 microns is optionally and preferably used for theseapplications.

One major problem in using scaffolds for tissue growth is in insuringappropriate elasticity of the scaffold both during tissue growth andafter such growth has been largely accomplished. The situation is likethe case of a broken bone—immobilization is desirable during the healingprocess, but it is desirable that normal mobility and elasticity returnsafter the healing process has satisfactorily progressed. The twistednanofiber yarns provide this tunability of elasticity if the initialscaffold material is impregnated with a host material (such asrelatively rigid bio-absorbable polymer), whose bio-regulated absorptionfrees the nanofiber yarn to have the elasticity associated with normalbody function and mobility.

Because nanotube yarns are electrically conducting, mechanically strong,flexible, and chemically stable, they can be utilized for implantablebiomedical devices such as implantable sensors and the inductor coilsfor implantable radio transmitters and transponders. Prior arttechnologies for using carbon nanotubes as biochemical sensors are welldeveloped, as are demonstrated strategies for obtaining nanotube sensorselectivity and selectivity. Since it is also well known in theliterature that functional neurons readily grow on carbon nanotubes,carbon nanotubes yarns and sheets could be used as a high-efficiencyelectronic interface to axons. The benefit of using carbon nanotubeyarns and sheets for these applications, as opposed to other types ofnanotube sheets and yarns, is in the combination of giant accessiblesurface area, high mechanical strength, high electrical conductivity,the absence of required binding agents, and the small obtainable yarndiameters and sheet thicknesses.

For some types of nanofiber synthesis and processing methods, whichresult in an undesired tendency for blood to clot on the nanofibers ornanofiber assembly, it is useful to coat the nanofibers with anothermaterial to prevent clotting. Amorphous carbon is one useful materialthat can be employed to prevent or reduce the clotting of blood. Otheruseful materials are proteins known in the prior art.

(k) Application of Nanofiber Sheets and Woven Textiles for AddressingIndividual Elements in Two and Three-Dimensional Arrays

The surprisingly high electrical anisotropy that the inventors observefor the electrical conductivity of nanotube sheets leads to another typeof invention embodiment, wherein these sheets are used for addressingselected regions (or pixels) in two-dimensional or three-dimensionalarrays. Example 23 shows that this electrical anisotropy varies frommoderate values of typically 10-20 for densified carbon nanotube sheets,to higher values of about 50-70 for undensified sheets, to arbitrarilyhigh values for either densified or undensified carbon nanotube sheetsthat have been appropriately pre-stretched orthogonal to the drawdirection to increase the anisotropy of nanotube electricalconductivity.

In one type of such invention embodiments, two of these highlyanisotropic sheets are placed parallel, with an orientation between thehigh conductivity directions in these sheets of 0. This angle betweenthe orientation direction in the two sheets is optionally between about30° and 90°, and optionally and more preferably about 90°. These sheetsare separated by one or more coatings or layers providing an effectiveresistance between nanotube sheets that is much higher than for thecurrent path along the nanotube sheets, so that the main reason forspreading of the current path in the sheets is due to deviation of sheetanisotropy from infinity.

The means to selectively provide a voltage at a desired position (pixel)in the material separating the nanotube sheets is provided by attachingelectrical contacts along at least one lateral side of each sheet. Thesecontacts are preferably spaced as a linear array that is at leastapproximately orthogonal to the nanotube orientation direction in eachsheet. The spacing between electrical contacts on a lateral sheet sideis measured by distance component orthogonal to the orientation.

This means for selectively addressing different pixels can be used forselectively addressing elements for various purposes, depending upon thenature of the resistive material that separates nanotube sheets.

The material at these different pixels, and separating the two nanotubesheets can be a sensor materials for (1) mechanical stresses (like apiezo-resistive material, a piezoelectric or polled ferroelectric, or adielectric providing a charged inter-sheet capacitance depending uponintra-pixel stress and strain), (2) local temperature, (3) aphotodetector material for local visible, infrared irradiationultraviolet, or higher energy gamma or particle radiations (based, forexample, for the material between the nanotube sheets, on photo-inducedconductivity, resistance change due to heating, or (4) an artificialnoise (for gas sensing) or artificial tongue (for liquid sensing) usingintersheet pixel area materials that are responsive to materials thatare in liquids or gases, especially including biological materials. Notethat the materials of (1) can be used to provide computer screens andelectronic textiles that enable data input using sensed changes inelectrical conductivity or electrical signals generated by touch.

The material separating the nanotube sheets in the pixel region can be amaterial or material assembly that either emits light or changes colorin direct or indirect response to a voltage applied to the pixel. Lightemission at a pixel at visible, infrared, and/or infrared wavelengthscan be achieved by various means, including fluorescence,phosphorescence, or incandescence. For example, the material separatingthe pixel region of the nanotube sheets can be the materials andmaterial assemblies used in the prior art for light emitting displays.Alternatively, the material separating a pixel region of the twonanotube sheets can be a resistive material that emits incandescentlight (or predominately incandescent light only in the infrared regionas a result of low temperature heating). Emission of infra-red light byfluorescent or incandescent means can be used to help reduce thevisibility of an article with respect to background for militaryapplications.

In other useful embodiments the high resistance material or materialassembly between the pixels of the two nanotube sheets can be one thatprovides a color change. Examples are those that provide a color changeas a result of heating (such as a thermochromic organic polymer, athermochromic inorganic material, or a thermochromic liquid crystal), aelectrochromic material (like a liquid crystal), or an electrochemicallychromatic material (like an electrochemically switchable conductingorganic polymer containing electrode attached to the first nanotubesheet that is separated by an electrolyte from a counter electrodeattached to the second nanotube sheet).

The responsive layer or coating between the sheets can be convenientlyapplied by various means, such as by coating the individual nanotubesheets with one or more layers before laminating the nanotube sheets.The material deposited as resistive material can vary in compositionacross the inter-sheet region, by deposition the inter-sheet material ormaterials in a spatially graduated manner, using the methods used tomake elements for combinatorial chemistry. These methods are especiallyuseful for making smart noses or smart tongues.

The space between two orthogonal sheets comprising oriented nanofiberscan also be usefully separated by air, other gas, or vacuum. A novelmatrix addressable bolometer using an air, other gas, or vacuum gap isillustrated in Example 54.

More than two nanotube sheets (or directly contacting nanotube sheetsstacks) can be used to provide three-dimensional pixilation when theresponsive material layer separates adjacent nanotube sheets (orneighboring nanotube sheet stacks). Each nanotube layer (or stack oridentically oriented nanotube sheets) can then be independentlyaddressed similarly to the case where there are only two nanotube sheetlayers (or contacting stacks of identically oriented nanotube sheetstacks).

The methods of these embodiments can also be applied when theneighboring non-electronically contacting nanotube sheets are replacedby two textile sheets comprising carbon nanotube yarns that areelectronically insulated with respect to intra-sheet or inter-sheetcontact (except in regions where the responsive material or responsivematerial assembly is located). The responsive material between thetextile layers can be provided by coating the responsive material orresponsive material layers onto the yarns before or after assembly ofthe nanotube yarns into the textile. If nanofiber yarns in a textilelayer go in more than one direction within a given textile layer, theseyarns in different directions should be substantially electronicallyinsulated with respect to each other than by mutual contact through aresistive responsive material or responsive material array.

These methods can also be applied to a single fabric sheet to obtain thedesired pixilated behavior by placing the nanofiber yarns in twodirections and insulating all yarns in the fabric by location orcovering so that there is no direct contact. The lowest resistancecontact between neighboring yarns should be through the electricallyresponsive material or material assembly. In such case the pixilatedresponse is provided by individually addressing the individual nanotubeyarns in the textile.

Such electrically anisotropic sheet and textile structures canoptionally be used for information storage, by utilizing resistiveresponse elements that undergo either permanent or reversible change inresistivity or capacitance as a consequence of ether an applied voltage(such as liquid crystal), a voltage-driven current flow (such as aconducting polymer electrochemical switch), or a combination thereof.For the permanent storage of information the responsive element can besimply and element that evaporates (opening the circuit) or carbonizes(closing the circuit) as a result of a pixel being addresses.

Alternatively, either permanent or reversible storage and retrieval ofinformation (or permanent record of localized radiation exposure) can beobtained by writing and or removing the information using radiation(such as light) and reading this information, and/or switching pixelresponse in the opposite direction using the pixel addressingcapabilities of the above sheet, textile, or optionally combined sheetand textile arrays.

These methods can optionally be applied to electronically conductingnanofiber yarns and electronically anisotropic nanofiber sheets that donot comprise carbon nanotubes or comprise carbon nanotubes incombinations with other nanofibers.

(I) Welding with Fusible Material and Surface Modification UsingSelective Nanofiber Heating at Frequencies in the Microwave, Radio,Ultraviolet, Optical, and Infrared Regions and by Electrical Heating

Example 30 shows polymer welding through heating of a transparent MWNTsheet that is sandwiched between plastic parts. The MWNT sheets stronglyabsorb microwave radiation, as evidenced by their use for this weldingof plastic parts in a microwave oven. In this example, two 5-mm-thickPlexiglas® plates were firmly welded together using the heating of asandwiched MWNT sheet to provide a strong, uniform, and highlytransparent interface in which nanotube orientation and electricalconductivity are maintained. The microwave heating was in a 1.2 kilowattmicrowave oven that operates at 2.45 GHz. FIG. 33 shows two 5-mm thickPlexiglas (polymethyl methacrylate) plates that were welded togetherusing microwave heating of a sandwiched MWNT sheet to provide a strong,uniform, and transparent interface in which nanotube orientation andsheet electrical conductivity is little changed. The combination of hightransparency and ultra-high thermal stability provide advantages notfound for the conducting polymers previously used for microwave-basedwelding. Among other applications, this microwave heating process can beused to make polymer composites from stacks of polymer sheets that areseparated by nanotube sheets, car windows that are electrically heated,and antennas in car window that have high transparency.

Welding of contacting fusible materials by selective heating ofnanofiber sheets can be similarly accomplished using selectiveabsorption of the nanofiber sheets at radio frequencies, infraredfrequencies, optical frequencies, infrared frequencies, ultravioletfrequencies, and combinations thereof and combinations with microwaveheating. Also, selective heating of a nanofiber sheet for weldingnanofibers with a fusible material can be accomplished by electricalcontact heating of the nanofiber sheet. The intensity and duration ofthe radiation-induced heating or the electrically induced heatingprocesses should be sufficient for at least partial fusion of thecontacted fusible material with the nanofibers. The intensity of theradiation and the electrical power delivered by electrical contactheating is preferably sufficiently high that liquidification of thefusible material occurs only locally in the contact region with thenanofiber sheet. Using region selective radiation delivery or regionselective electrical contact heating, or a combination thereof, for thenanofiber sheet, the degree of welding with fusible material ormaterials can be arbitrarily provided in an area selected manner.

The choice of fusible materials for these processes is littleconstrained. However, it is preferable that the fusible material hassufficiently low viscosity at the fusion temperature that it is abletoflow on the desired time scale needed for rapid processing. For thecase where the fusible material is an organic polymer, the uppertemperature of the fusion temperature is usually limited by the thermaldegradation temperature of the polymer. However, the fusible materialsfor welding processes are preferably those that do not have significantabsorption at the wavelengths used for heating. If the heating processis by electrical contact to the nanofiber sheet, the fusible materialshould be substantially electronically insulating.

The fusible polymers polycarbonate, polyvinylbutyral (marketed, forexample, under the trade name Salflex®), polymethyl methacrylate, andpolystyrene are especially preferred for the above described processesof inter-layer welding and the below described processes of surfacewelding. Also, inorganic and organic glasses are preferred.

For the case of welding using polyvinylbutyral, this polymer isoptionally and preferably sandwiched between panes of glass. Forprocessing using radiation induces heating of polyvinylbutyral, aglass/polyvinylbuttral/nanofiber/glass stack or a glass/nanofibersheet/polyvinylbuttral/nanofiber sheet/glass stack is preferably heatedby microwave of other means.

The above-described electrical heating means and heating means by theabsorption of radiation can be used to weld together sheets of fusiblepolymers and layers of fusible textiles (or a fusible sheet with afusible textile) at an overlap region of these yarns and textiles, wherea nanofiber sheet of invention embodiments is located.

Nanofiber sheets can be conveniently attached to the surface of aplastic or other fusible material by a related process, by sandwichingthe nanofiber sheet between a low melting polymer and a high meltingpolymer, selected so that only the low melting polymer melts as a resultof the temperature increase caused by radiation absorption or electricalcontact heating of the nanofiber sheet.

These surface and interlayer bonding methods enable the incorporation ofnanofiber-based antennas and heating elements in either the surfaceregion of a fusible material or between layers of fusible materials,such as layers of a window.

Other benefits of such surface bonding processes are desirablemechanical properties enhancements for the surface region and theelectrical conductivity provided by the nanofiber sheet. The surfaceenergy of the nanofiber-incorporated surface region depends on thedegree of heating generated infiltration of the nanofiber sheet.

When the degree of infiltration of the nanofiber surface area isincomplete, so that carbon nanofibers protrude from the surface, theresult of this surface treatment process can be to provide a highlyhydrophobic surface. Such a highly hydrophobic surface can be useful foravoiding water droplet condensation, which can be employed for avoidingfogging for optical elements.

The surface energy, and therefore the degree of hydrophobicity will beinfluenced by absorbed gases. This dependency can be eliminated and thesurface can be made either hydrophobic or hydrophilic by intentionallyabsorbing either hydrophilic or hydrophobic materials (or materialshaving mixed hydrophobic and hydrophilic character) on the nanofibers orbundles of nanofibers that protrude from the substrate.

In addition, the surface can be reversibly electrically tuned betweenhydrophobic and hydrophilic by placing two parallel, non-contactingnanofiber sheets in the surface region, so that one of these sheets incompletely imbedded and separated from the nanofiber sheet on thesurface by an ionically conducting layer. Application of a potentialbetween the two nanofiber sheets results in electrochemical charging ofthe nanofiber sheets and corresponding changes in sheet surface energy,which determines wetability. A suitable assembly for this purpose ofobtaining tunable charges is wetability can be made by placing ananofiber sheet upon the surface of a fusible material (or attaching itbe other means if the surface material is not usefully fusible),layering upon this nanotube sheet a fusible solid-state electrolyte andthen a nanofiber sheet (which then becomes the outer surface layer).Compression of this stack with a another material (that does not causefusion or undesirable absorption of the actinic radiation used forradiation-based nanofiber heating or an electrical conductivity thatinterferes with electrical contact based welding) enables fabrication ofthe targeted two electrode outer layer separated by electrolyte when theactinic radiation or electrical contact heating is applied to causefusion.

(m) Use of Substrate-Supported Nanofiber Sheets for the PatternedDeposition of Oriented Nanofibers

Applicants find that localized mechanical stress can be used to transfer30 nm or thinner layers of oriented nanofibers from one substrate (thetransfer substrate) to another (the receiving substrate) as a patternedarray that maintains nanofiber orientation.

For instance, Example 34 and FIG. 37 show that substrate-supportedcarbon nanotubes in a 30 to 50 nm thick carbon nanotube sheet on onesubstrate can be mechanically transferred to produce a printed imagehaving about the same thickness on another substrate. This transferoccurs without substantial loss of nanotube orientation. Placing thesubstrate-supported nanofiber sheet face down on standard writing paperand writing on the supporting non-porous paper using a sharp object, thenanotube sheet was transferred from the surface of a non-porous paper tothe regular paper. The picture on the left in FIG. 37 shows the nanotubesheet attached to the substrate (non-porous paper) after the transferprocess and the picture on the right shows ordinary writing paper withthe transferred image.

Most important, optical microscopy on the transferred nanotube sheetregions shows that the nanotube alignment present on the originalnanotube coated sheet is maintained in the nanotube pattern transferredto the porous paper. Hence, the orientation of nanotubes in transferredcircuit patterns can be controlled at will by varying the relativeorientation between the image producing sheet (the transfer substrate)and image receiving sheet (receiving substrate).

In the results of FIG. 37, the nanofiber sheet has been densified by theliquid treatment densification method of Example 23 before the nanotubetransfer step. Other results in Example 34 indicate that an undensifiednanotube sheet can similarly be used for printing patterned arrays oforiented nanofibers that have not been densified. This process is lessattractive than the one using densified sheets, since the nanotubeswhere transferred to portions of porous paper that were not under thewriting instrument. Nevertheless, the nanotubes that were transferredwere much more firmly bound to the porous paper than those that wereaccidentally transferred, so the later could be easily brushed awaywithout disturbing the intentionally transferred nanotubes.

This process has general applicability for nanofiber sheets that areoriented with respect to nanofiber direction. These results show thateither densified or undensified nanofiber sheets can be used fordepositing patterned arrays of oriented nanofibers.

As an alternative to using locally applied stress to provide patterneddeposition of nanotubes from a nanotube sheet onto another substrate(the receiving substrate), the receiving substrate can be patterned(such as by lithographic or mechanical means) to have raised and loweredsurface regions. Application of a uniform stress to the transfer sheetor other transfer substrate can then be used to transfer the nanotubesto the raised surface regions of the receiving substrate.

In another invention embodiment, selective area transfer (i.e.,patterned transfer) of nanotubes from the transfer substrate to thereceiving substrate can be achieved by depositing a patterned array offusible material (such as polymer) on the receiving substrate. Contactof the transfer substrate with the receiving substrate, followed byheating to the fusion point and subsequent cooling, transfers thenanotubes from the transfer substrate to the receiving substrate. Thisheating can be optionally accomplished by microwave or radiofrequencyheating of the nanotube sheet.

Such patterned arrays of deposited oriented nanofibers can be used fornanofiber circuit element fabrication, such as electronic wires andinter-connects, antennas, resistors, capacitors and supercapacitors.

Also, such transfer of nanotubes from the transfer substrate to thereceiving substrate can optionally be repeated more than one time, suchas to increase the thickness of deposited nanotubes in selected areas.Also, additional transfer, processes between transfer and receivingsubstrates can be used for reducing part of all of the anisotropy of aregion of transferred material. For instance, this can be done bychanging the orientation between transfer and receiving substratesduring successive printing processes. In addition, a first depositionprocess can be followed by a coating process of the entire receivingsubstrate (or any portion of it) with another material (such as a solidstate electrolyte for a film supercapacitor or a dielectric for anordinary capacitor). This second coating process can then be followed byadditional transfer of nanotubes from the transfer substrate (or asecond transfer substrate) on top of the material provided by the secondcoating process. Such methods can be repeated at will to developmultilayer structures.

(n) Nanofiber Sheet Appliqués

Example 31 shows that a transparent carbon nanotube sheet can beattached to an adhesive film as an electrically conducting layerproviding microwave absorption and the electrical conductivity neededfor resistive heating. In addition, this example shows that the adhesiveon adhesive films can extrude though the nanotube sheet to provide highadhesion of the laminated adhesive film/nanotube sheet to metal, glass,plastic, and other surfaces. When attached to a flexible substrate (suchas a polymer film), this example (and FIG. 34) shows that thetape/nanotube sheet/plastic sheet can be repeatedly bent to high angleswithout causing a significant change in the in-plane resistance of thenanotube sheet.

Example 42 describes equipment for the manufacture of nanotube sheetsattached to adhesive tape. Examples 50, 51, and 52 show means forstoring nanotube sheets after fabrication and these means can be usedfor subsequent attachment of the nanotube sheets to adhesive sheets.

(o) Nanofiber Sheet Filters

Simultaneously achieving high filtration rate and the ability to filtervery small particles, such as viruses, bacteria, and nanometer scalecolloidal particles, is ordinarily problematic for filtration of bothgaseous and liquid media. The problem is that small pore sizes providelow filtration rates, especially if the filter is thick. Whilefiltration rate can be increased by decreasing filter thickness, thepressure differential between opposite sides of the filter must then bereduced because of possible rupture of the filter when the filterthickness is small—which partially eliminates the rate advantage ofusing a thin filter membrane.

The solid-state drawn nanofiber sheets of invention embodiments can helpsolve this problem by providing filtration membranes that are so strongthat filter membrane rupture is less of a problem even when thenanofiber filter is very thin.

The present invention embodiments provide a filter structure comprisinga solid-state drawn nanofiber sheet or sheet laminate having small poresize is attached on at least one size by a thicker membrane havinglarger pore size. The average pore of the thicker membrane is chosen tobe at least two times smaller in largest dimension than the typicalnanofiber length.

If the nanofiber sheets are highly oriented, two or more sheets can beoptionally laminated with crossed directions of nanofiber orientation sothat low lateral strength orthogonal to the orientation direction in onesheet is reinforced by the high strength in the orientation directionfor a laminated sheet.

A greatly enhanced effective load-carrying capability of the nanofibermembrane preferably results from the spanning of individual nanotubesacross the contacting pores in the supporting filter structure. Thebenefit of this arrangement is that the resistance to filter rupture isdetermined by the strength of the individual nanofibers, which aregenerally much higher than that of the sheet. Hence, the filtration ratebenefit of using very thin sheet thicknesses can be achieved withoutincurring the risk of membrane rupture when a high pressure drop isapplied across the filtration membrane.

The porous material supporting the nanofiber sheet membrane should be onthe low pressure side of this membrane, and can optionally be chosen totrap larger size particles. For purposes of membrane cleaning byapplication of a reversely directed pressure difference across themembrane, the nanofiber sheet membrane can optionally be supported onboth sides by a thicker membrane having much smaller pore size.

The supporting membrane or membrane with larger pore size can be any ofthe various conventional membrane types, and can optionally comprisefibers or nanofibers. Alternatively, the supporting membranes can be asheet material that contains holes or channels, such as a metal wiregrid, metal plate containing holes, or a sheet of anodized aluminum.

The nanofiber membrane preferably comprises carbon nanotubes and theseat least one membrane in a possible membrane stack is preferablyassembled by a processing step of this invention that involvessolid-state assembly (such as the processes of Example 21, 50, and 52).The solid-state fabricated sheets are preferably densified, such as bythe liquid-based densification process of Example 23. However,especially for the purpose of gas filtration, such as air filtration,the nanofiber sheet can optionally comprise undensified nanofibersheets.

It should be understood that this invention embodiment is applicable toboth planar and non-planar filters. A particularly preferred non-planarmembrane is a cylindrical or conical membrane. Also, nanofiber basedmembranes can be conveniently used for cross flow filtration, whichminimizes filter clogging by passing the liquid to be filteredtangentially to the filter membrane.

(p) Additional Applications of Nanofiber Sheets and Ribbons asTransparent Conductors

The applications of solid-state drawn carbon nanotube sheets (andribbons) as transparent conductors are diverse, and are in some casesfacilitated by nanofiber sheet strength, toughness, microwave absorptioncapability, polarized emission and polarized absorption capability,tunable work function, extreme flexibility without degradation ofelectrical and thermal conductivity, and nanotube sheet porosity.

Some of these are applications modes described in the Examples: Example14 on transparent substrates and electrodes for optically monitorablecell growth, Example 25 for transparent sensors having low noise and alow temperature dependence of electrical conductivity, Example 29 on atransparent source of polarized incandescent light, Example 30 on atransparent layer for microwave welding of plastic and for microwavebonded transparent coaling layers (for EMI shielding, antennas, andheating elements), Example 31 on transparent appliqués, Example 32 ontransparent elastomeric electrodes, Example 33 for transparent organiclight emitting diodes, and Example 34 for transparent printed circuitelements.

The transparency in combination with electrical conductivity is alsoespecially useful for making transparent and low visibilitysupercapacitors, such as planar sheet supercapacitors of those that areconformal to a surface. Such transparent or low visibilitysupercapacitors preferably comprise two nanotube sheets that areseparated by an electrolyte.

Sheet transparency is also useful for using carbon nanotube sheets andribbons for electromagnetic interference (EMI) shielding. For instance,the transparent nanotube sheets can be used to provide EMI shielding foroptical displays, such as computer screens.

Also, both transparent nanotube ribbons and non-transparent nanofiberribbons (optionally helically wrapped) can be used for the outer EMIshielding covering for coaxial wires and cables, wherein the interiorregions of the wire of cable contains the signal or power transportingcomponent and the nanotube EMI covering is separated from this innersignal or power transporting component by an electronically insulatinglayer. EMI shielded wires of such type are especially useful forapplication in electronic textiles (where they can provide thecombination of low visibility and structural reinforcement of thecoaxial wire).

The combination of electrical conductivity and transparency for nanotubesheets and ribbons is also usefully employed for heating elements thatare transparent. These heating elements can be employed in electronictextiles for clothing applications, in automotive and aerospace vehiclewindows, and for electrically heated furnaces and ovens (where thebenefit is to provide high visibility for the heated contents of thefurnace or oven). A substrate can be usefully and optionally employedfor such transparent electrical heater for furnace or oven and saidsubstrate is optionally a glass or quartz substrate.

Emitted light from the heating element of high temperature ovens canobscure the view of the furnace contents. This problem can be mitigatedby using the polarized nature of light emission from oriented nanotubesheet or yarn heating elements (see Example 29). By placing a polarizingsheet or material with polarization dependent reflectivity between theviewer and the incandescent nanotube emitter, the visibility of furnacecontents can be improved. The direction of preferential light absorptionor reflection should optionally and preferably be parallel to the majorpolarization direction of the light emission from the nanotube sheetheating element (which is parallel to the nanotube orientationdirection). The benefit of using a polarizing element that accomplishespolarization by largely non-absorptive means by reflecting light andinfrared radiation back into the oven or furnace is to increase oven orfurnace efficiency.

Transparent carbon nanotube sheet or ribbon coatings on windows,eyeglasses, and like devices (such as binoculars) can optionally beoriented so that the polarizing effect of the nanotube sheet minimizesglare. This can be done by orienting the nanotube alignment direction ofthe nanotube sheet so that it is at least approximately horizontal.

The use of transparent nanofiber sheets as one or more electrodes fornewspaper-like displays, and other related displays, is also important.In such applications the nanotube sheets of invention embodimentsprovide the advantages of flexibility without loosing conductivity andtransparency. The substrate for these potentially single-sheetnewspapers can optionally be sheet materials like those for ordinarynewspapers (or materials having similar properties), and updating thepages of these pages can potentially be based on currently availableinformation, which could be transmitted wirelessly.

The chromatic change used for these newspapers can be those of ananofibers sheet electrode in the newspaper, a liquid crystal basedcolor change, a thermally driven color charge of a thermochromicmaterial, an electrowetting driven color change, an electrochemicalcolor change of a component in one of the electrodes (includingelectrochromic nanoparticles or an electrochromic coating contacting thenanofiber electrode), inter-electrode particles that rotate in responseto an inter-electrode field to provide the color change, inter-electrodeparticles that rise or fall to provide the needed color change inresponse to an inter-electrode field, or by other means known on theart. An example of the use of electrowetting for an electronic paperdisplay is provided by R. A. Hayes and B. J. Feenstra in Nature 425, 383(2003).

The chromatic display of invention embodiments can optionally comprise awhite, a near white, or a suitably colored first substrate upon which anelectronically conducting, an optically transparent electrode that iseither directly or indirectly attached to the first substrate, a secondtransparent electrode, and a material or material system that eitherdirectly or indirectly provides an electrically driven chromaticresponse, wherein at least one of the said transparent electrodes is ananofiber electrode. This nanofiber electrode is optionally preferably ananotube comprising electrode.

A nanofiber electrode in electronic newspaper and like chromaticdisplays can be optionally comprised or nanofiber sheets, ribbons,yarns, woven and non-woven yarns. These electrodes can optionallyprovide the major mechanical support for the display, such as anelectronic newspaper, or can be a layer that is supported by anothermaterial or material assembly.

The details of the transmission of information to the newspaper(typically by wireless communication) are not the subject of thisinvention, and means are known in the art. However, the inventorsdemonstrate herein transparent nanofiber sheet electrode properties thatare usefully employed for such electronic newspaper.

These electronically conducting, transparent nanofiber sheets canreplace the brittle metal oxide sheets of the prior art forelectrochemically provided text described, for example, by U. Back etal. in Advanced Materials 14, 845 (2002). These transparent,electrically conducting sheets. can also replace the electrodes used forthe tunable-wetting based displays described by R. A. Hayes and B. J.Feenstra in Nature 425, 383 (2003).

Example 52 describes means wherein nanofiber sheets can be attached tocellulose-based papers and sheets, and these methods are genericallyapplicable for these invention embodiments.

(q) Elastomerically Deformable Nanofiber Sheets

Examples 32 and 90 demonstrate a method wherein transparent carbonnanotube sheets can be transformed into highly elastically deformableelectrodes, which can be used as electrodes for high-strain artificialmuscles and for conversion of high strain mechanical deformations toelectrical energy, and for the tunable dampening of large amplitudemechanical vibrations. The illustrative actuator material is a siliconerubber.

These elastomeric nanotube sheets can undergo over 100% change indimension will substantially maintaining in-plane resistance. As shownin FIG. 35, the initial sheet resistance of the obtained unloadedsilicone rubber/MWNT sheet composite was 755 ohms/square. However, afteran initial increase in resistance by ˜6%, the resistance changed lessthan 3% during the subsequent four strain cycles to 100% strain.

Ordinary conductors cannot undergo nearly such large strains withoutlosing electrical contact with the actuating material. While conductinggreases are used to maintain electrical contact to electrostrictiveactuator materials that generate 100% or higher strains (R. Pelrine, R.Kornbluh, Q. Pei, and J. Joseph, Science 287, 836 (2000)), these greasesare not suitable for use as electrodes for stacks of electrostrictivesheets that can generate large forces and high strains without requiringseveral thousand volt applied potentials.

Further experimentation (see Example 32) has shown the generalapplicability of this method of providing highly elastomeric electrodeson an elastically stretchable textile substrate. For example, attachmentof an undensified nanotube sheet (prepared as in Example 22) to a 120%elongated elastomeric Spandex® fabric (by pressing and by subsequentlyapplying the liquid-based densification process of Example 23) resultsin a nanotube electrode materials that can be elastically relaxed andre-stretched repeatedly to the initial elongation without undergoing asubstantial resistance change. Suitable Spandex® fibers and/or textilesare made by DuPont (and called Lycra® fibers and Spandura® textiles),Dorlastan Fibers LLC, INVESTA, and RadiciSpandex Corporation.

For applications requiring a large range of elastic deformability for ananofiber sheet or ribbon, the stretch of the elastomeric sheet beforenanofiber deposition is preferably at least 4%, more preferably at least10%, and most preferably at least 100%.

Woven textiles that are suitable as elastomerically deformable materialsubstrates for elastic deformable nanotube sheets include those that arehighly elastically deformable in one in-plane direction (like Spandura®)and those that are highly elastomerically deformable in two in-planedirections (like Tru-Stretch®, which is a Lycra® and Nylon® blend). Thedifference between these different elastomerically deformable textilesis well known in the art and is largely due to the ways that thesetextiles are woven and/or the chemical formulations of the componentfibers or yarns.

This method of converting poorly elastically deformable sheets andribbons to highly elastically deformable sheets and ribbons can bepracticed for various sheets and ribbons comprising a host of differenttypes of nanofibers and nanoribbons, some of which are described inSection 2. It is optionally preferable that these nanofibers and ribbonshave a length of above about 10 microns, and more optionally preferablyabove 100 microns. Also, the ratio of nanofiber or nanoribbon length tonanofiber or nanoribbon thickness in the thickest lateral dimension isoptionally preferably above 100, more optionally preferably above 1000,and most optionally preferably above 10,000.

This described method of making nanofiber sheets and ribbons elasticallydeformable by attaching the nanofibers to an elastomerically stretchedsubstrate can also be used for nanofiber and nanoribbon sheets that havelittle or no nanotube alignment in an in-plane direction.

Also, the initial stretch of the elastomerically deformable (orquasi-elastomerically deformable) substrate can be a biaxial stretch.

In addition to being applicable to porous elastomeric textiles (seeExample 32), these methods for making nanofiber sheets and ribbonselastomerically deformable can be practiced for porous elastomer sheets(like porous silicone rubber sheets).

The nanofibers can optionally be directly formed as a sheet on apre-stretched porous elastomeric sheet by filtration of nanofibersdispersed in a solution or super critical fluid. In such process aprestretched porous elastomeric sheet can optionally serve as a filterfor nanofiber deposition. In such case, well known methods of formingaligned or unaligned nanofibers as sheets can be optionally employed.

The nanofiber sheets or ribbons suitable for the practice of theseinvention embodiments include the various methods of the prior art (seereferences in the Section on Description of the Background Art) formaking sheet-shaped nanofiber arrays. These methods include variousphysical methods, such as solution filtration or magnetically assistedfiltration methods, deposition from a volatizable liquid (such as byspin coating), coagulation of nanofibers dispersed in solution,deposition of nanofibers at a liquid-gas interface (such as by aLangmuir-Blodgett deposition process), deposition of nanofibers from aliquid crystal assembly in a liquid, deposition by centrifugation of ananofibers dispersed in a liquid, shear deposition of nanofibersdispersed in a liquid, deposition of a nanofiber aerogel sheet(including such aerogel sheets formed directly or indirectly by CVD),and by deposition from a nanofiber dispersion in an acid or super acid.

The nanofibers or nanoribbons can be formed on a pre-stretchedelastomeric substrate by any chemical methods known to producenanofibers or nanoribbons, including, for example, CVD or plasmaassisted CVD.

The nanotubes grown on or laminated with an elastomeric substrate can bea nanofiber forest. In such case the nanofiber forests can be grown onor laminated with an elastomeric substrate while the elastomericsubstrate is either stretched or unstretched. Such application ornanotube forests on elastomeric substrates enables the controllabledeformation of the elastomeric substrate so that an unspinable forestbecomes spinable.

Example 90 shows that an elastomerically deformable nanotube sheet canbe made by the process of Example 32 and then overcoated with a secondelastomeric silicone rubber sheet, while retaining the ability of thenanotube sheet to be elastomerically deformed without producing asignificant dependence of nanotube sheet resistance on elongation of thenanotube sheet. The importance of this demonstration is that it enablesthe fabrication of high strain actuator stacks comprising more that onelayer of actuator material (such as an electrostrictive rubber likesilicone rubber) and more than two electrodes.

This process can be conveniently extended to produce elastomericallydeformable stacks comprising one or more nanotube electrode sheets thatare laminated between sheets of elastomer, wherein the number ofalternating nanotube sheet electrodes and elastomer sheet electrodes isarbitrarily large.

The method of this example and Example 90 can be used to produceinflatable balloons containing one or more layers of conducting nanotubesheets. To start the process of conducting balloon formation, theinitial inner balloon layer can optionally be inflated using a gas orliquid or formed on a mandrel will in non-inflated state, andsubsequently un-expanded before application of the first nanotube sheet.

(r) Elaboration on Artificial Muscle Applications

Electronically conducting nanofibers in yarns of invention embodimentscan be used as either (a) a material whose dimensional changes providesactuation, (b) a material that delivers the electrical energy and otherelectrical effects that causes actuation-producing dimensional changesof another material, and (c) any combination of (a) and (b).

According to these differing natures of the origin of actuation, thenature of the nanofiber yarn used for actuation preferably changes. Morespecifically, highly twisted or highly coiled nanofiber yarns arepreferred for applications in which the material providing thedimensional changes is not the nanofiber yarn.

A first example is a nanotube yarn in which the function of thenanotubes in a twisted yarn is to provide electric heating that causesanother material to produce actuation. In this case, the twistednanofiber yarn should optionally and preferably be highly twisted sothat it provides a reduced contribution to yarn stiffness in the yarndirection and the possibility of long reversible dimensional changes inthe yarn direction. In this case the helix angle of twist (measured withrespect to the yarn direction) is preferably above 50° and morepreferably above 700.

As an alternative to using a yarn that is highly twisted in order reducethe effects of yarn mechanical stiffness on displacements caused by anactuating material, the nanofiber yarn can be suitably plied so that theorientation of the nanofibers in the plied yarn is at a large angle withrespect to the direction of the yarn. FIG. 3A shows this achieved effectof yarn plying on providing such large angle orientation.

Materials suitable for undergoing large dimensional changes when heatedare well known in the art. These include, for instance, the Veriflx™polymer of Cornerstone Research Group Inc. (see E. Havens et al.,Polymer Preprints 46, 556 (2005)), thermoplastic polyurethanes, such asthe aromatic polyester based thermoplastic polyurethane called Morthanethat is available from Huntsman Polyurethanes (see H. Koerner et al.,Polymer 46, 4405 (2005), H. Koerner et al., Nature Materials 3, 115(2004), H. M. Jeong et al., 37, 2245 (2001), and H. Tobushi et al. 5,483 (1996)), liquid crystal elastomers (see J. Naciri et al.,Macromolecules 36, 8499 (2003), D. K. Shenoy et al., Sensors andActuators A 98, 184 (2002), and D. L. Thomsen, et al., Macromolecules34, 5868 (2001)), biodegradable shape memory polymers (see A. Lendleinand R. Langer, Science 296, 1673 (2002)), shape memory polymer networksbased on oligo(ε-caprolactone) (see A. Lendlein et al., Proceeding ofthe National Academy of Sciences, 98, 842 (2001)), shape memory polymersbased on cross-linked polycyclooctene (see C. Li et al., Macromolecules35, 9868 (2002)), and hydrogels that display shape memory behavior (see,for example, the poly(vinyl methyl ether) hydrogels in R. Kishi et al.,Journal of Intelligent Material Systems and Structures 4, 533 (1993)).

The highly twisted electrically conducting nanofiber yarns of inventionembodiments provide a means for electrically heating such shape memorypolymers to obtain actuation of the shape memory material while at thesame time insuring by using a high yarn twist that the yarn does notexcessively interfere with yarn direction actuation. The shape memorypolymers are preferably infiltrated in the nanofiber yarns or overcoatedon them, or both infiltrated and overcoated.

The shape memory polymer contacting the nanofiber yarn can optionallyinclude other conducting aids, such as conducting nanofibers orparticles (like carbon black), or combinations or conducting particlesand nanofibers. These optionally used conducting nanofibers,nanoparticles, or combination of conducting nanofibers and nanoparticlesis preferably electrically percolated and electrically contacting thetwisted nanofiber yarn.

The elastomerically deformable nanotube sheets of Section 10(q) can alsobe used to provide electrical heating that causes actuation of shapememory polymers. The elastomerically-deformable nanotube sheets areoptionally and preferentially attached on opposite sides of a shapememory polymer sheet. Electrically heating these two nanofiber sheets toprovide at least approximately the same level of power dissipation (perunit area) for both sheets is usefully employed for obtaining uniformchanges in the length (or length and width) of a shape memory sheet. Onthe other hand, electrically heating the two nanofiber sheets onopposite sides of the shape memory polymer sheet to different extents(or electrically heating only one of these sheets can be used to providecantilever type bending of the shape memory sheet.

As an alternative to using a highly twisted nanofiber yarn as anelectrode for such actuators in which yarn direction as the actuationdirection, the nanofiber yarns of the present invention embodiments canbe helically wrapped around the actuating material. Optionally, theactuating material can be infiltrated into such helically wrappednanofiber yarn. The benefit of helical wrapping, and especially hightwist helical wrapping, is to limit the mechanical effect of thenanofiber yarn on the actuation direction stroke of the actuatingmaterial.

The electrically conducting highly twisted nanofiber yarns of inventionembodiments and the elastomerically deformable nanotube sheets ofSection 10(q) can also serve as electrode materials for artificialmuscles in which gel actuator materials or conducting organic polymerspredominantly provide actuation. This gel actuator response can, forexample, be caused by electrochemically generated pH changes of a liquidelectrolyte that causes the expansion or contraction of the gel in partbecause of changing degree of gel hydration. Polyacrylonitrile is anoptionally preferred material for the gel actuators. Methods for usingthis polymer are described by H. B. Schreyer in Biomacromolecules 1, 642(2000). Suitable conducting polymers as the predominately actuatingmaterial are described by R. H. Baughman in Synthetic Metals 78, 339(1996) and in numerous other literature reports.

(s) Applications as a Heat Pipe

The nanofiber yarns can be braided or more simply configured as highperformance heat pipes that combine high strength and high toughnesswith high effective thermal conductivity.

These heat pipes will function like conventional heat pipes, in thatheat transport is largely a result of evaporation of the working fluidat the hot end of the heat pipe to absorb thermal energy and thecondensation of the working liquid at the cold end of the heat pipe torelease thermal energy. The nanofiber yarn functions to wick the workingfluid between the hot and cold ends of the heat pipe.

FIG. 105 schematically illustrates a heat pipe that uses a hollowbraided carbon nanotube yarn wick (10501), which is sealed with respectto loss of the working fluid using an overcoat (polymer or ceramic) thatserves as an outer diffusion barrier (10502) or by incorporation of theyarn heat pipe into a matrix.

The nanofiber wick material can be optionally chemically or physicallytreated so that wetting occurs for the working liquid of the heat pipe,or so as to enable wetting of only the outer region of the nanofiberwick, and thereby enabling the inner region to function as a gastransport region. Using this strategy, plied nanofiber yarns can beconveniently used as the wick. The result is that one component of thenanofiber wick provides wicking and a second component of the wick (andspace external to unwetted yarn components) provides the vapor path. Thefluid and vapor transporting yarn components in a nanofiber heat pipecan optionally be fundamentally different yarns, like MWNT yarns andKevlar® or Spectra® fibers.

Various well known working fluids like water and methanol can be used,which have a useful working range of −5 to 230° C. and −45 to 120° C.,respectively. For very high temperature application materials likepotassium or sodium as the working liquid, which have working ranges of400 to 800° C. and 500 to 900° C., respectively.

Less than 20 μm diameter heat pipes can be made using these approaches,although much larger diameter heat piper are more conveniently used formost applications. The smaller diameter yarn heat pipes could be woveninto structural textiles, and could be incorporated into a structuraltextile or as individual yarn fibers into a resin to make a yarn heatpipe/polymer matrix composite.

(t) Nanofiber Sheets and Yarns as Highly Anisotropic Thermal andElectrical Conductors and Sensor Arrays

The nanofiber yarns, sheets, and ribbons of invention embodiments canprovide high electrical conductivity, high thermal conductivity, andhigh thermal diffusivity, as well as high anisotropy for these transportproperties. These properties provide important applications embodiments.

For example, carbon nanotube sheets of invention embodiments can providethe following unique properties and unique property combinations usefulfor conductors of heat, temperature and electrical current: (1) highstrength and toughness; (2) high electrical and thermal conductivities;(3) high absorption of electromagnetic energy that occurs reversibly;(4) low temperature coefficient of resistance; (5) very low 1/f noise,where f is the frequency of AC current, which makes such conductors verygood transmission lines; (6) high resistance to creep; (7) retention ofstrength even when heated in air at 450° C. for one hour; and (8) veryhigh radiation resistance even when irradiated in air.

Example 11 provides measurement results showing the high electricalconductivity of the carbon nanofiber yarns. The high electricalconductivity and high anisotropy of carbon nanotube sheets of inventionembodiments is shown by the results in Example 23, where it is alsoshown that sheet resistance changes little as a result of liquid-baseddensification of a nanofiber sheet by a factor of over 300, though theelectrical conductivity changes by about this factor of over 300 as aresult of the decreased cross-section that results from densification.

An extremely high thermal diffusivity (D of above 0.1 m²/s) and a highthermal conductivity of solid-state drawn MWNT nanofiber sheets (K=50W/mK) of the present invention (see preparation in Example 21) allowsfor rapid highly anisotropic transfer of temperature fluctuations tocolumn and/or row electrodes of a matrix addressable sensor with minimalenergy dissipation (see Example 54).

The low thickness and density of free-standing carbon MWNT sheets ofinvention embodiments make the very attractive for bolometric materials,as shown in Examples 54 and FIG. 54. The main advantages of a MWNT sheetas a bolometric material are: (1) an extremely low heat capacity (lowinertia); (2) a high absorbance coefficient over a wide wavelengthrange, i.e., 0.2-20 μm; (3) an emissivity coefficient close to unity (asfor graphite); (4) a high degree of flexibility; (5) resistance toradiation damage; and (6) suitability for use in high magnetic fields.At the same time, the low thermal coefficient of resistivity (TCR),α=−7.5×10−4 K−1 is a limiting factor for thermal sensitivity in suchmaterials. It is, however, an advantage for other sensor applicationsand for resistors with low temperature dependence.

For a position sensitive bolometer, an array of highly aligned nanofibersheets can be fabricated by various means (for details see Example 54).For example, this can be done by (a) drawing highly-aligned nanotubesheets from the side of a carbon nanotube forest and (b) attaching thenanotube sheets to two sides of a substrate, wherein such a substrate isa solid or flexible substrate comprising an array of metallic electrodeshaving a rectangular opening in a central region so as to formorthogonally-oriented suspended sheets on opposite sides. Each of theseside electrodes can be covered with a temperature-sensitive thin film.In one embodiment, a semiconductor film, e.g., VO₂ or other materialwith high temperature coefficient of resistivity is deposited onmetallic electrode pads so as to form serial resistance with a carbonnanotube interconnected network. The serial resistance of the carbonnanotube network and semiconductor films between column and rowelectrodes is measured in order to create a thermal image of a radiatingobject. In other embodiments, the metallic pads are covered withcomplimentary pairs of thermoelectric materials, e.g., iron on one sideand constantan alloy on the other.

The application of high nanofiber sheet thermal conductivity anddiffusivity for electronic part cooling is described in Example 59.

Section 10(k) has shown how the high electrical anisotropy of nanofibersheets of invention embodiments can be used for various applications,including pixilated sensors.

(u) Nanofiber Yarns and Textiles for Application as Cold ElectronCathodes for Field Emission of Electrons

Nanofibers, and in particular carbon nanofibers, are well known to beuseful as electron field emission sources for flat panel displays,lamps, gas discharge tubes providing surge protection, and x-ray andmicrowave generators (see W. A. de Heer, A. Chatelain, D. Ugarte,Science 270, 1179 (1995); A. G. Rinzler et al., Science 269, 1550(1995); N. S. Lee et al., Diamond and Related Materials 10, 265 (2001);Y. Saito, S. Uemura, Carbon 38, 169 (2000); R. Rosen et al., Appl. Phys.Lett. 76, 1668 (2000); and H. Sugie et al., Appl. Phys. Lett. 78, 2578(2001)). A potential applied between a carbon nanotube-containingelectrode and an anode produces high local fields as a result of thesmall radius of the nanofiber tip and the length of the nanofiber. Theselocal fields cause electrons to tunnel from the nanotube tip into thevacuum. Electric fields direct the field-emitted electrons toward theanode, where a selected phosphor produces light for a flat panel displayapplication and (for higher applied voltages) collision with a metaltarget produces x-rays for the x-ray tube application.

Cold electron cathodes that rely on electric field emission from carbonnanofibers of either SWNTs or MWNTs are well known and have alreadyfound commercial applications (Carbon Nanotubes: Synthesis, Structure,Properties, and Applications. Topics in Applied Physics, 80,Springer-Verlag, Heidelberg, 2000, pp. 391-425]. The threshold emissionvoltage for CNT cold cathodes is quite low: 1-3 V/μm (J.-M. Bonard, etal., Appl. Phys. A 69, 245 (1999)), particularly when compared to Si- orMo-micro-tip cathodes (50-100 V/μm (C. A. Spindt J. Appl. Phys. 39, 3504(1968))). Additionally, current densities of such CNT cold cathodes canbe as high as 10⁸ A/cm² for SWNT cold cathodes, and comparably high forMWNTs cold cathodes (Y. Cheng, O. Zhou, C. R. Physique 4, (2003)). Theprospects for applications of CNT field emitters in displays are verybright (R. H. Baughman, A. A. Zakhidov, and W. A. de Heer, Science 297,787-792 (2002)), and several prototype devices comprising this uniquematerial have already been fabricated. Namely, CNT-based electron sourcehave been used in fluorescent lamps (A. K. Silzars, R. W. Springer,PCT/US96/13091, J.-M. Bonard, et al., Appl. Phys. Lett. 78, 2775 (2001),N. Obraztsov et al., Appl. Surf. Sci. 215, 214 (2003)), X-ray tubesbased on CNT-cathodes (G. Z. Yue, et al., Appl. Phys. Lett. 81(2), 355(2002)), and flat panel displays (N. S. Lee, et. al., Diamond andRelated Mater. 10, 265, (2001), W. B. Choi et. al., Technol. Dig. SID.(2000)). Motorola recently developed a CNT-TV and Nano Proprietary Inc.announced a CNT cathode-based TV of 25 inches. Also, CNTs may findapplication in high-resolution electron microscopy as very bright pointelectron sources with low energy spread (de Jonge et al., Nature, 420,393 (2002)).

For applications requiring low threshold voltages and high currentdensities from large area cathodes materials, the lifetime of existingcold cathodes is relatively short and the homogeneity is insufficientfor high resolution display applications.

A critical problem hindering applications of these carbon nanotubes(CNT) cold cathodes is the need for methods of assembling thesenanotubes into the framework of a macroscopic mounting system that issufficiently strong and suitably shaped such that the properties of theCNTs for field emission can be effectively utilized.

Addressing many of the above-described limitations of the prior art, insome embodiments, the present invention is directed to nanofiber yarncold cathodes, methods of making said yarn cathodes, and to applicationsof said yarn cathodes. Additional embodiments provide for makingpatterned structures of yarns, such as alpha numeric code and specialsymbols (shown in FIG. 71) and weaving yarns into “cathode textiles”(shown in FIG. 77). The yarns for use as cold cathodes can be made ofnot only multiwall carbon nanotubes, but also of single-wall carbonnanotubes and other diverse nanofiber material (such as conductingnanofibers and nanoribbons described in Section 2).

Most importantly, the yarn cathodes are easy to assemble in variousarchitectures such as a single approximately vertical yarn tip (or anarray of yarn tips) emitting from the end tip (FIGS. 66 B and 73). Insuch geometry the yarn (7301) is raised approximately vertically by thestrong electric field lines (7303) which are concentrated on the tips offree ends of nanofibers (7302) at the cross section of the yarn. Whenvery high voltages are applied, currents emitted by such numerous tipsis so high, that light is emitted by an overheated yarn end, as shown inFIG. 68.

Another geometry for cold field emission from yarn is to place the yarnhorizontally on a planar surface, so that emission occurs from the sideof the yarn (FIGS. 66 A, 70 and 72). In FIG. 72, the nanofibers on theside of the yarn (7201) which is closest to an anode (7204) protrudetheir free ends (7202) raised by the force from the lines of electricfield (7203) coming from the anode and concentrated by the tips at theends (7202).

Moreover, the sides of yarn can be engineered to have an extended“hairy” structure, in which individual nanofibers and larger assembliesof nanofiber (nanofiber bundles and assemblies of bundles) extend fromyarn sides (such as the sides of a twisted yarn).

The Inventors find that such hairy structure can be produced bymechanical treatments, chemically based or physically based chemicaltreatments, plasma treatments, thermal treatments, and electrical fieldtreatments.

Electric field treatments are especially useful for providing hairynanofiber yarns which can be specially engineered with numerousnanofibers and small diameter assemblies of nanofibers whose free endsextend out of the yarn. Data shows the effects of such electric fieldtreatment on improving electron emission characteristics (see currentcurves in FIGS. 69 and 76 and the SEM micrographs and schematicillustrations in FIG. 74).

The desired hairy yarn structure develops during operation of thecathodes in a high electrical field, due to forces created by strongelectric field which eventually pull the ends out of the yarn body andeventually partially unwrap the yarn (shown schematically in FIG. 74 Cand the SEM micrographs of FIGS. 74 A and 74 B).

Other sub-architectures can be envisioned, such as making multi-yarnplies, and singles yarns and plied yarns with various knots along theyarn having the ability to modulate electron emissive properties in theknot region (as shown at FIGS. 64 and 75). It has been demonstratedthat, at least in some cases, the knotted regions of yarn cold cathodesemit fewer electrons (see FIG. 75 and examples herein).

Some invention embodiments described herein provide novel fabricationmethods, compositions of matter, and applications of nanofiber yarns ascold cathodes having quite useful properties. For example, carbonnanotube yarns of the present invention provide the following uniqueproperties and unique property combinations for cold field emission coldcathodes: (1) high current density that can be controlled by the numberof twists in the yarn and by the number of singles yarns in a plied yarncathode; (2) very low threshold electric field, in the range below 0.5.V/micron, which may self-improve with operation time (FIGS. 69 and 76);(3) low operational voltage, which is below 300 V at the distance of 400micron between cathode and anode; (4) self-improvement of cathodes interms of current and voltage performance increase with cycling (FIGS. 69and 75); (5) high thermal stability; (6) high mechanical strength andvibration stability; (7) the possibility to pattern cathodes by knottingthem (FIG. 75) and by other methods, including patterning of theoriginal nanofiber forest, from which the yarns are twist spun; and (8)very high radiation resistance, including electron beam and UV radiationstability).

In order to increase the number of nanotube fibers on the yarn surfacethat are available for field-enhanced electron emission, the nanofibersin the convergence zone for spinning can optionally be perturbed by theapplication of electric or magnetic fields, air flows, sonic or ultrasonic waves, and combinations thereof. As a result of this perturbation,some of the nanofibers are incompletely incorporated in the yarn duringtwist, so that they extend laterally from the yarn surface (FIGS. 65 and74). The hairy part 7403 in FIG. 74 has additional free ends ofnanotubes, which are protruded from the body of a twisted yarn (7402),wrapped around a wire 7401. The result is a “hairy yarn” in which thenanotube hairs that extend from the yarn provide enhanced field emissionof electrons (7404) due to field emission form ends (as shown in FIGS.69 and 76).

Hairy yarns for field emission of electrons can also be produced byapplication of false twist (i.e., a twist that is counterbalanced by anequal and opposite twist), which disturbs the nanofibers in the spunyarn. Additionally, the nanotube yarn surface can be intentionallyabraded after spinning has been completed by mechanical or chemicalprocesses or a combination thereof. The nanofiber yarns of the inventionembodiments can also be usefully employed as electron thermal emissionsources (known as hot cathodes), which differ from the cold cathodeelectrode emission sources in that resistive heating is used to enhanceelectron emission.

One further way to produce a “hairy yarn” cold cathodes for enhancedfield emission is to feed nanofibers from a subsidiary forest into theconvergence point (where twist is produced during yarn spinning) so thatthe nanofibers from the subsidiary forest tend to be incorporated normalto the yarn surface. With more ends available for field emission, suchtreatment would increase the intensity of electron emission.

The herein demonstrated mechanical robustness of the twisted carbonnanotube yarns, the small yarn diameter, and the yarn geometry, whichpermits a useable fraction of the nanotube fibers to extend from thefiber surface, so as to provide field enhancement effects, providesadvantages of the twisted nanotube yarns for this application. Forexample, the yarn geometry can be usefully employed as the electronemitting element for an x-ray endoscope for medical exploration or as acentral electron emission element for a cylindrically shapedhigh-intensity light source, where the emission phosphor is on acylinder that is external to and optionally approximately coaxial withthe central nanofiber singles yarn or plied yarn (as shown at FIG. 78 ofa prototype phosphorescent lamp with yarn cathode). Carbon nanotubes areoptionally and especially preferred for nanofiber yarns applied for thisfield emission application.

(v) Optically Transparent Cold Electron Cathodes Comprising NanofiberSheets for Use in Displays and Lamps

Carbon nanotube cathodes are essentially cold cathodes, which may beextensively used in applications where low power consumption and narrowenergy spread of emitted electrons are required. The carbon nanotubeemitters rank among the best electron field emitters currentlyavailable. Additionally, carbon nanotubes are very robust, chemicallyinert, and may be successfully used in such devices under the requisitevacuum or inert gas conditions. Relevant for applications needing lowthreshold voltages and high current densities from large area cathodes,impressive current densities have been obtained.

All such prior art cold cathodes are not optically transparent, sincethey are made either of micro-tips of metals like Mo (see C. A. SpindtJ. Appl. Phys. 39, 3504 (1968)), semiconductors or conventional carbonfibers/nanostructures, all of which are used as thick materials that areoptically opaque.

However, most applications of cold electrons involve the production oflight which needs to be able to escape the device, be it a display or acathodoluminescent light source. For example, in conventional fieldemission flat panel displays (N. S. Lee, et. al., Diamond and RelatedMater. 10, 265, (2001), W. B. Choi et. al., Technol. Dig. SID (2000)),light emanates from a phosphor screen that is coated by Al to collectthe charge. Thus, some of the electrons are absorbed by the Al layer,which significantly erodes the efficiency of the display (which is shownin FIG. 81). Also, high voltage must generally be applied to allowelectrons to penetrate the Al layer, which is also problematic. Apossible solution is to use a screen glass with ITO and a low voltagephosphor coating. This way, a lower bias voltage may be applied. In thiscase, however, almost half of the light is emitted back to the cathodeand lost (as shown in FIG. 82). Hence, the efficiency of this type ofdisplay is also not high.

With the transparent carbon nanotube (CNT) sheets of the presentinvention, another display configuration becomes possible. It utilizes acarbon nanotube transparent sheet as the cathode, glass coated withmetal having a high optical reflectance (e.g., Al), and a phosphormaterial, shown in FIG. 83. Thus, almost all light emitted by thephosphor layer upon electron impact from the nanotube cold cathode willbe guided back (e.g., by an Al mirror) towards the transparent cathodeand the efficiency will be considerably improved. In the meantime, a lowvoltage regime will be utilized, due to nanoscale field emission.

The above-described transparent carbon nanotube sheets may be used ineither flat panel field emission displays (FEDs), such as thoseextensively developed by Motorola and Samsung, or in a new type of highefficient, low power consumption, flat backlight light sources forliquid crystal displays (LCDs). In the latter case, shown schematicallyin FIG. 84, it is particularly important that the light, which passesthrough the transparent carbon nanotube sheets, is partially polarizedin a direction perpendicular to the carbon nanotube orientation (asdescribed in M. Zhang, S. Fang, A. Zakhidov, S B. Lee, A. Aliev, K.Atkinson, R. H. Baughman, Science, 309, 1215 (2005)). This may be anadditional technological advantage because, once developed, noadditional polarizer will be needed in front of the thin film transistor(TFT) matrix.

It has also has been suggested recently in the art that field emissiondisplays with transparent cold cathodes (if such devices could becreated) would provide additional advantages such as increasedbrightness, increased efficiency, and lower field voltages (U.S. Pat.Nos. 5,646,479; 6,611,093; 6,777,869; 6,933,674; 6,943,493; 6,914,381;and 6,803,708). Images formed by such displays can be viewed from bothsides of the field emission display panel and multiple displays can bestacked together. However, in the above-referenced patents, thehypothetical emitting material is opaque and the light is able totransfer through the cathode only by means of a specially-designedoptical system and a special electrode with a plurality of holes. Incontrast, embodiments of the present invention allow for the creation ofnanofiber-based highly optically transparent cold cathodes which do nothave the above-described disadvantages.

As described previously, methods are known for growing both single-walland multi-walled nanotubes as forests of parallel aligned fibers on asolid substrate and for utilizing MWNT forests as electron coldcathodes. However, the resulting forest assemblies have variousinstabilities at large current loads, one such instability being theflash evaporation of the catalyst and carbon (discovered recently bypresent inventors: A. A. Zakhidov et al., J. Appl. Phys. (submitted)),followed by spark emission of light and transfer of CNTs from thecathode to the anode, which thereby destroys the cathode.

Advances have been made in creating robust forests of oriented CNTs onglass substrates (Motorola, Samsung). Other methods of making coldcathodes from CNTs include formation of a composite with polymericbinder in which CNTs are not oriented, but random. Nevertheless,impressive emissive properties have been obtained for polymer-binderSWNTs. A problem with polymer binder, however, is that the nanotubes arenot present in sufficient quantities in the polymer to effectivelycontribute to electron field emission and also to such properties asthermal and electrical conductivities. Additionally, the uniqueelectrical properties of the individual nanotubes are diluted, since themajor component of the cathode is by far the polymer binder.

In some embodiments, the present invention is directed to opticallytransparent nanofiber sheet cold cathodes, methods of making saidnanotube sheet cathodes, and to applications of said nanotube sheetcathodes. Importantly, the yarn spinning and the sheet and ribbonfabrication technology described herein is applicable for producingyarns, ribbons, and sheets of various nanofibers and nanoribbons ofdiverse materials (e.g., WS₂, WO₂, etc.) for use in a variety ofelectron field emission applications.

As described above, processes for making carbon nanotube sheets for coldcathodes comprising nanofibers generally comprise the steps of: (a)arranging nanofibers in an array selected from the group consisting of(i) an aligned array; and (ii) an array that is converging towardsalignment, so as to provide a primary assembly (known as a forest); (b)drawing the free-standing self supporting sheet of nanofibers from aforest or other manifold (see above description of non-forest spinning);and (c) depositing the said nanofiber sheet on the transparent support.In some embodiments, the nanotube sheets comprise carbon nanotubes.

Such carbon nanotube sheets and ribbons of the present invention provideunique properties and property combinations favorable for field emissionby large area transparent cathodes, such as tunability by doping orchemical modification to provide a tunable work function; flexibility,strength, and toughness; resistance to failure; high electrical andthermal conductivities; high absorption of energy that occursreversibly; very high resistance to creep, retention of strength evenwhen heated in vacuum at above 1000° C. for long periods, and very highradiation and UV resistance (particularly in vacuum where cold cathodesmostly operate). In some embodiments, the nanofibers are nanoscrolls. Insome embodiments, the nanofibers are chemically and/or physicallymodified before or after a fabrication of the nanofiber ribbons orsheets.

Densification of a CNT sheet of the present invention, which can enhancethe adhesion of cold nanofiber sheet cathodes to substrates, can be doneby dipping a substrate comprising a CNT sheet (or other nanofiber sheet)into inorganic or organic liquids, such as ethanol, methanol, oracetone, and then permitting drying. Such densification is useful forpreventing the CNTs from flying off the substrate in a strong electricfield. In some embodiments, after densification, all of the CNT sheet'sborders were covered with a carbon-conducting tape (e.g., such as thatused for SEM samples) to provide electrical contact to the nanofibersheet, avoid side effects, provide mechanical attachment, and for otherpurposes, as shown in FIG. 79. The carbon nanotube transparent sheet(7901) is placed on glass substrate (7902), pristine or coated with alayer of ITO. The edges of the sheet are covered with SEM tape (7903)with the aim to avoid edge effects. The typical emitting area is 50 mm².

These unfavorable side effects can include strong electron emission fromthe nanofibers mechanically raised at sides, protrusion at the ends,elevation by high electrical field, and such side effects stronglyinfluence the emission uniformity because of a large field enhancementfactor on the protruded edges.

In some embodiments, in order to increase the number of nanotube fiberson the transparent cathode sheet surface that are available forfield-enhanced electron emission, the nanofibers in the convergence zone(of the drawing process) can be optionally perturbed by the applicationof electric or magnetic fields, air flows, or sonic or ultrasonic waves.As a result of this perturbation, some of the nanofibers areincompletely incorporated into the sheet during the drawing process suchthat they extend laterally from the sheet surface. The result is a“hairy surface” of the sheet, as shown schematically in FIG. 80 (top andbottom portions), in which the nanotube hairs that extend from the sheetprovide increased electric field and hence enhanced field emission, asdescribed in the caption to FIG. 80. In FIG. 80 (top portion) theelectric field lines (8001) go from the anode (8002) to the ends of tips(8003) and sides (8004) of single nanofibers within the sheet. FIG. 80(bottom portion) shows the schematics of field electron emission fromtips (free ends extended from the sheet) and from sides of nanofibers(within the sheet). Electrons are shown as dots (8005) emitted from tips(8006) and sides (8007) of single nanofibers within carbon nanotubesheet.

Hairy sheets for the field emission of electrons exist to some extent inthe originally-drawn nanofiber sheets, while a number of free ends ofnanofibers or nanofiber bundles, making up a hairy surface, can beadditionally enhanced by application of a plasma which disturbs thenanofibers in the drawn sheets. Additionally, the nanotube sheet surfacecan be intentionally abraded after drawing has been completed, whereinsuch abrasion is carried out by mechanical and/or chemical processes.The field emission from sides of nanofibers, also contribute a lot inthe total current of sheet cold cathode (as depicted in FIG. 80).Although the threshold field is larger for side emission, the totalcurrent density from the sides can be significantly larger than from theends of hairy sheets. The nanofiber sheets of the invention embodimentscan also be usefully employed as hot cathodes, i.e., thermionic electronemission sources, which differ from the cold electrode emission sourcesin that resistive heating is used to enhance electron emission, even insmall electric fields.

The transparent nanofiber sheet cathodes are particularly suitable foruse in various phosphorescent displays, since they permit various typesof architectures for such displays, as shown at FIGS. 81-84. FIG. 81shows, schematically, a non-transparent cold cathode in a conventionalgeometry having the cathode (8102) on the glass or other substrate(8101) placed on the back side of the display, i.e., behind thephosphorescent screen (8105). Since the charge collector is an Al mirrorfilm (8104) on the back side of the phosphorescent screen, wherein someemitted electrons are lost while penetrating through the Al thincoating, before creating light in the screen (8105) In this architectureall light is radiated forward (8106) (since it is reflected from Al).

On the other hand FIG. 82 shows, schematically, a cold nanofiber cathodein another conventional geometry having a cold cathode (8202) on theback of the display, i.e., behind the phosphorescent screen (8205).Charge collector is a transparent ITO film (8204) and in this geometrysome light is radiated backwards (8207) from the phosphorescent layerand it is captured inside the device and cannot escape from the display,therefore creating various problems, such as decreasing the contrast andresolution, causing photoelectrons and similar problems.

Another favorable and promising architecture is shown in FIG. 83 whichschematically illustrates a new transparent nanofiber cold cathodedevice design of present invention. In this novel architecture thetransparent cathode (8303) is in the front of the display and electronsare emitted backwards from the viewer and towards the phosphorescentscreen (8305) on the back of the display. The light emitted by screen8305 and reflected by back anode plate (8306) is all transferred toward(8302) to the viewer after passing through the transparent cathode(8303). And because nanotubes in the cathode are all aligned, the lightthat passes through the aligned nanotubes is polarized normal to theorientation direction of the nanotubes of the transparent cathode.

The new type of a polarized back-light source is possible, as shown inFIG. 84, which schematically illustrates how the transparent nanotubecathode can be used for generating polarized light for a conventionalliquid crystal display (LCD).

The flat light source of the design of FIG. 83, is placed on the back ofthe LCD display in such a way, that light passes through the transparentnanotube cathode into the LCD. In FIG. 84, the nanotube cathode 8407 onglass support 8411 emits electrons through vacuum toward the whitephosphorescent screen (8409) placed on another glass substrate (8411)coated with anode (8410). The generated light is polarized (which isdesirable for LCD operation), since the transparent cold cathode acts asa polarization filter due to highly oriented nanotubes. This lightpasses through components of LCD: the alignment layers (8404), thin filmtransistors (8405), the liquid crystal layer (8412), color filters(8402) and second polarizer (8401). Such design eliminates a polarizerin the LCD part, since it is incorporated in the back-light source.

The herein demonstrated electrical and structural robustness of thetransparent carbon nanotube ribbons and sheets permits one to depositsheets on flexible and elastomeric substrates, such as plastics, rubber,very thin glass, metallic foils and similar, in such a way that thiscreates flexible cold cathodes which can be further processed in desireddevice geometries by changing their shape in accordance with the neededgeometry. The flexibility of sheet cold cathodes still permits a useablefraction of the nanotube fibers within the sheets to extend from thesheet surface so as to provide field enhancement effects and providesadvantages of the nanotube sheets for this application as flexible,elastomeric, and other shape tunable cold cathodes.

For example, the flexible sheet cold cathode geometry can be usefullyemployed as the electron emitting element for a flexible x-ray endoscopefor medical exploration or as a flexible transparent electron emissionelement for a flexible screen high-intensity light source, where theemission phosphor is optionally on another flexible surface that isexternal to and optionally conformal with the flexible nanofiber sheetcathode, so that vacuum spacing exists between these two flexiblesurfaces. Carbon nanotubes are particularly suitable as nanofibers fornanofiber sheets on flexible substrates for this field emissionapplication.

(w) Flexible Organic and Polymeric Light Emitting Devices (OLED andPLED) Using Optically Transparent Nanofiber Sheets as Charge Injectors

Organic optoelectronic devices, such as organic light emitting devices(OLEDs) similarly to organic photovoltaic cells (OPV), such as plasticsolar cells based on P3HT (poly-(3-hexylthiophene)) with fullerenederivative PCBM ([6,6]-phenyl-C61 butyric acid methyl ester), need atransparent hole-collecting or hole injecting electrode to replace thepresently used ITO (indium-tin-oxide) and other transparent conductiveoxide (TCO). Such TCOs are widely used in various applications where theoptical transparency is needed along with high electrical conductivity.However, these TCOs are brittle and easily damaged when bent, even whenin the form of very thin coatings.

The prospects for applications of CNTs as charge injectors for OLEDs hasbeen previously discussed, but no reports exist on the use of nanotubesas transparent hole injectors. Recently, however, Rinzler and co-workershave reported on the application of transparent single-wall nanotubeelectrodes as electrodes in inorganic, p-type GaP light emitting devices(A. G. Rinzler, S. Pearton, Semiconductor Device and Method UsingNanotube Contacts, PCT Patent Publication No. WO2005083751). Nanofibersand nanotubes have been used as a component of a composite layer inOLEDs by others with the aim of increasing the conductivity of thelayers, but not as a transparent electrode. Prior art electron injectorsare not optically transparent, since they are made either of micro-tipsof metals (e.g., Mo), semiconductors (e.g., Si), ornon-optically-transmissive carbon nanostructures. Since mostapplications involving charge injection are for the production of light,such light must be able to escape the device, be it a display or a lamp.

A critical problem hindering applications of these CNT transparentcharge injectors and collectors is the need for methods for assemblingthese nanotubes into macroscopic and mechanically strong structures andshaped articles that effectively utilize the properties of the nanotubecharge injection or collection.

Methods are known for making both single-wall and multi-walled nanotubesas mats and films and also for making carbon nanotube sheets by solutioninfiltration methods, and for utilizing such non-transparent MWNT aselectrodes for charge collectors in optoelectronics. However, theresulting nanotube films and sheets are nontransparent and theefficiency of devices based on such films is poor (as in photovoltaiccells, described by H. Ago et al., Adv. Mat. 11, 1281 (1999)).

Accordingly, in some embodiments, the present invention is directed totransparent nanofiber sheet charge injectors for light emitting devices,methods of making said nanofiber sheet charge injectors, both anodes andcathodes, and to applications of said nanofiber sheet charge injectorsin light emission devices. Additional embodiments provide for thedrawing of nanofiber sheets and ribbons having very high internalinterfaces and a three-dimensional network of nanofibers, and theirincorporation with a light emissive material component of an OLED.

Importantly, this technology of sheet and ribbon drawing is applicablefor making sheets and ribbons of diverse nanofibers and nanoribbons ofdiverse materials for use in a variety of LED applications. Relevant forapplications needing low threshold voltages and high current densitiesfrom a large area, suitable devices have been generated for a variety ofarchitectures for OLEDs and PLEDs.

In some embodiments, the nanofiber sheets comprise carbon nanotubes.Optionally preferred sheet compositions comprise either carbon singlewalled nanotubes, carbon multiwalled nanotubes, doped versions of thesenanotubes, or combinations of these nanotube materials.

FIG. 85 describes an optionally preferred architecture for a polymericLED that uses a transparent carbon nanotube (CNT) electrode made by thesolid-state draw process of Example 21. The CNT sheet (8505) was placedon a substrate (8506) of high-quality display glass (Corning 1737 usedfor active-matrix liquid crystal displays) and densified using a polarsolvent (either ethanol or methanol) and the method of Example 23.Polymeric films were deposited from solution using spin-casting.Multiple layers (8505) of the hole-injection layer PEDOT:PSS weredeposited from an aqueous solution and baked at 120° C. for 30 minutesafter each layer deposition. The first layer was deposited at a highspin rate (6100 rpm) with high acceleration (21000 rpm/s). This isbelieved to “flatten” the nanotubes. Subsequent layers were alsodeposited at 6100 rpm, but with a slow acceleration—so that thickerlayers were obtained. Layer 8503 of emissive polymer (MEH-PPV) was spunfrom a chloroform solution (˜0.2 wt % polymer in chloroform) at a speedof ˜3000 rpm. Cathodes (thin metallic layers, patterned as desiredpixels) were deposited using thermal evaporation of calcium (8502) andaluminum (8501) under a vacuum <2×10⁻⁶ Torr.

The inventors found that light emission was suppressed when thePEDOT:PSS was eliminated in the above, suggesting that excitons can bequenched on the nanotubes.

FIG. 86 A shows another type of organic LED that uses a transparentnanofiber sheet electrode (8606) made by the process of Example 21 andemploys vacuum deposition of low-molecular weight organic molecules. ThePEDOT:PSS hole transport layer (8605) was again deposited from anaqueous solution on top of nanotube layer (8606) that was densified on aglass substrate (8607) using the method of Example 23. The remainingorganic layers (hole-transport layer, α-NPD (8604) and emissive/electrontransport layer Alq₃ (8603) were deposited under vacuum using thermalevaporation. Both materials were deposited a rate of 1 Å/s and at apressure below 2×10⁻⁶ Torr. The layer thickness was 700 Å for the α-NPDand 500 Å for Alq₃. A bilayer cathode (10 Å lithium fluoride (8602)under 1200 Å aluminum film (8601)) was deposited on top of the organiclayers using thermal evaporation.

FIG. 87 shows a novel type of OLED/PLED architecture that uses thetransparent nanofiber electrode of Example 21. This device employs abottom-up construction, which is possible due to availability offree-standing transparent nanofiber sheets of the present invention.Since this nanofiber sheet is transparent, the OLED/PLED light emissioncan be through this sheet. Hence, the last element in this bottom-updeposition process can be the carbon nanotube sheet electrode.

The device fabrication process involves first depositing anon-transparent cathode layer (a double metal cathode: 8705/8704) on asubstrate and subsequent deposition of the polymeric layers (such as aMEH-PPV type emissive layer (8703) and a PEDOT:PSS hole transport layer(8702)). The final deposited layer, the transparent carbon nanotubesheet anode (8701), was placed on the device either by means ofpressure-induced process (stamping, which transfers the nanofiber sheetfrom an original substrate to the device) from another substrate or bylaying a free-standing nanotube sheet.

Such bottom-up construction is necessary for certain applications,specifically those which incorporate drive electronics on a siliconwafer (e.g., active matrix and thin-film transistor displays). In suchcases, the cathode layer cannot be deposited on an existing PLED orOLED. The deposition of a transparent hole injector, such asindium-tin-oxide as the last deposited layer requires high temperatures,which can damage the predeposited polymeric or molecular device layers.Therefore, a low-temperature or room-temperature process is desired toprevent damage or morphological changes in the organic layer. Thedescribed mechanical transfer of transparent nanotube sheet meets thisneed and can provide an ideal solution for such display applications.

In another invention embodiment using the transparent nanotube sheets ofExample 21, the OLED is made completely transparent so that light canpass through the entire device. FIG. 89 shows such a transparent PLEDwhich uses a carbon nanotube sheet as both the anode (8904) and thecathode (8901). As a result, both electrodes and the device itself aretransparent.

The above device can be built on a flexible/elastomeric substrate(8905), thereby realizing the ultimate goal of a flexible/elastomericdisplay in which several OLEDs can be stacked on top of each other. Thedevice structure comprises a liquid-densified nanotube sheet (8904) onsubstrate (8905) and subsequent polymeric layers (8903 and 8902)deposited by spin-casting. The patterned nanotube cathode (8901) is thenplaced on the structure, either by stamping or other transferringmethods of invention embodiments. This last nanotube sheet cathodeshould be coated with a low work function material (for example calciumor a bilayer of calcium/aluminum). A low work function metal is neededat the interface for creating efficient electron injection.

An alternative, and possibly more efficient, process for placing thecathode sheet on the device involves drop-casting the MEH-PPV film,rather than spin-casting it. The corresponding solution with dissolvedpolymer is placed on the substrate, which is then turned upright so thatexcess solution runs off. The free-standing nanotube sheet (8901) isimmediately placed on the wet film (8902). The film is then allowed todry in an inert atmosphere. This procedure produces an improvedinterface between the MEH-PPV and the transparent nanotube sheet.

Transparent carbon nanotube sheets of the present invention provideunique properties and property combinations for charge injectionelectrodes, such as high work function combined with extreme toughness,resistance to failure of bended parts, high electrical and thermalconductivities, and very high radiation and UV resistance, even whenirradiated in air. Furthermore these nanotube sheets can be drawn asvery wide, free-standing self-supporting sheets and films that can belaminated by various methods on the top of OLED architectures, changingthe existing bottom-up fabrication methods to inverse top-to-bottom typemethods.

The nanofiber sheets of the present invention can therefore be used in avariety of diverse optoelectronic applications. In some embodiments, thedrawing of transparent electrodes as free standing films is extended forproducing sheet and ribbon electrodes of nanofibers and nanoribbons ofdiverse materials, such as WO₃ or MoO₃ or MoS₂ nanofibers. This expandsthe range of LED applications.

Applications of the nanofiber sheet and ribbon electrodes of the presentinvention include an entire family of optoelectronic devices, OLEDs,PLEDs, OFETs, field effect transistors (FETs) with transparent gates,electrochemical devices such as nanofiber-based batteries, fuel cells,artificial, and electrochromic displays.

In some embodiments, the present invention provides novel fabricationmethods, compositions of matter, and applications of nanofiber sheetsand ribbons having quite useful properties for application as lightemitting devices, e.g., OLEDs and PLEDs. For example, carbon nanotubeyarns of the invention embodiments provide the following uniqueproperties and unique property combinations useful for OLEDs: (1) highwork function, preferable for hole injection into hole transport layerof OLED, (2) high internal surface and porosity for interfacing withother functional materials of the device, (3) high mechanical strength,toughness, and resistance to damage by bending, permitting use inflexible devices and electronic textiles, and (6) very high radiationand UV resistance, even when irradiated in air.

(x) Transparent Nanofiber Sheets and Yarns as Charge Collectors andSeparation Layers for Photovoltaic Cells and Photodetectors

Transparent electrically conducting electrodes are needed that aremechanically strong, highly flexible, and self-supporting. Also, it isuseful for these electrode materials to have extended internal surface,so that they can most effectively interface on the nanoscale with otherfunctional materials, such as organic electronic materials andnanoparticles (such as quantum dots and rods).

It is also beneficial for the transparent electrically conductingelectrode materials to have tunable work functions. Also, theavailability of electrode fabrication means for obtaining eitherisotropic or highly anisotropic electrode properties is useful,especially when the highly anisotropic properties are electricalconductivity, thermal conductivity, and optical transparency.

Some embodiments of the present invention feature an electrode thatcomprises nanofiber sheets of invention embodiments, such as thesolid-state fabricated transparent carbon nanotube sheets of Example 21.The transparent nanofiber sheet electrode (TNSE) can be designed for usein an optoelectronic device, such as photovoltaic cell.

In other embodiments, the present invention provides a solid filmphotovoltaic cell that includes the TNSE electrode as part of itsfunctional architecture, thereby contributing to photogeneration ofcharge carriers.

In some embodiments, the present invention provides a photovoltaic cellwhich includes the TNSE electrode, a second electrode and an organicsemiconductor or conjugated polymer or a composite between theelectrodes.

In additional embodiments, the present invention provides amultijunction or tandem photovoltaic cell, which includes several TNSEelectrodes separating the single junction parts as transparentseparation layers (also known as charge recombination layers, orinterconnect layers) placed between each separate single junctionphotocell part.

The present invention is also directed to the application of transparentcarbon nanotubes and nanofibers, in combination with nanofiber yarnswoven into textiles, as either effective three-dimensional ortwo-dimensional charge collection and charge recombination layers inorganic (i.e., plastic, excitonic, or hybrid) solar cells and tandem(i.e., multi-junction) solar cells. The TNSE electrodes can be anodesand/or cathodes as top electrodes and as charge recombination layers intandem.

Charge collecting electrodes of the present invention are in someinstances anodes (i.e., hole or plus charge collectors), which have manyadvantages over prior art TCOs: they are flexible, mechanically strong,have anisotropy of optical and electrical properties, and have astructure of a three-dimensional network, being especially favorable forcollecting charge in bulk heterojunction solar cells. Moreover, pristineCNT transparent electrodes have large work functions (5.1-5.3 eV), whichis higher than the work function (w.f.) of ITO (4.7 eV), but which isfavorable for collection of plus charges or holes.

Applicants also put forth methods to use the transparent carbonnanotubes sheets and ribbons as electron collectors or cathodes (byusing special coatings and chemical modification methods). Alsodescribed herein are methods of making said transparent carbon nanotubesheets, ribbons and twist spun yarns as parts of a photoactive solarcell architecture, contributing to photogeneration of charge carriers.Additional embodiments provide for the spinning of free-standingnanofiber ribbons having arbitrarily large widths. Importantly, thetechnology of these flexible transparent electrodes for solar cells canbe extended to produce various sheets and ribbons from nanofibers andnanoribbons of diverse materials for use in a variety of solar celltypes and applications.

Applications for nanofiber sheet and ribbon electrodes of the presentinvention include various solar energy harvesting textiles, solar cellsfor hydrogen production, and combined solar energy harvesting with fuelcell batteries (as described in examples herein).

Invention embodiments described herein provide novel architectures,processing and fabrication methods, compositions of matter, andapplications of transparent nanofiber sheets and ribbons havingproperties and functionalities useful for solar cell design. Forexample, nanofiber sheets, and particularly carbon nanotube sheets ofinvention embodiments provide the following unique properties and uniqueproperty combinations, particularly useful for solar cells: (1) highoptical transparency, (2) low electrical sheet resistance, (3)three-dimensional topology of the mesh-like CNT network, which allowscharge collection from a large volume, and not only from a planarinterface, like for usual ITO electrodes, (4) an extended interface ofthe three-dimensional network, which enhances charge separation andcollection, (5) high thermal conductivities and thermal diffusivities,which provide heat dissipation in solar cells, (6) high work functionrequired for collection of plus charges, holes in solar cells, (7) highflexibility as opposed to the brittle character of ITO and otherinorganic TCOs, (8) very high resistance to creep, (7) interpenetratingcontinuous morphology of the nanofiber network, as opposed to cermettype non-percolated morphology of nanoparticle electrodes, which isfavorable for collection of charge carriers from the bulk heterojunctiontypes of architectures, (8) retention of strength even when heated inair at 450° C. for one hour, and (9) very high radiation and UVresistance—even when irradiated in air.

In some embodiments, nanofiber electrodes are coated first from afree-standing sheet on a flexible or other substrate (as the anode orplus collecting electrode) and then covered or impregnated byphotoactive layers, which are bulk heterojunctions of organicsemiconductors with donor-acceptor interfaces favorable for chargeseparation. Then, the cathode is deposited on the top to finalize adevice.

In another architecture, first the non-transparent cathode electrode isdeposited on a substrate. Then, the photoactive layer or layers or bulkheterojunction interpenetrating network is coated from a liquid phase bydipping, spin coating, ink jet printing, screen printing, or by vacuumdeposition. Finally, the transparent nanofiber electrode is placed onthe photoactive layer from a free-standing, self-supporting sheet stateinto a deposited sheet state by placing the sheet onto the surface andthen pressing by stamping methods described below in more detail.Alternatively, the nanofiber sheet of ribbon can be placed on the deviceby transfer from another surface, such as a carrier tape.

The TNSEs can be highly transparent or semi-transparent depending on thenumber of sheets and their thickness. The light absorption properties ofTNSEs can be adjusted by the length of individual nanofibers within theself-assembled nanostructure, based on the antenna-effect of quarterwavelength and properties of a directional radiation. Additionally, thestrength of light absorption in the photoactive part that is impregnatedinto the three-dimensional nanofiber electrode can be increased by aneffect of an enhancement of local electric field of a photon in thevicinity of nanofibers by coating them with appropriate metal orsemiconductor nanoparticles or by creating core-shell structures inwhich plasmonic effects are additionally enhanced

In some embodiments, the TNSE can further include at least one organicfunctional polymer, such as poly-alkylthiophene (PAT) or PEDOT-PSS, incombination with other material, such as fullerene derivative PCBM. TheTNSE can be a composite of several nanofibers, such as a composite ofmultiwall and single wall nanotubes, added together in order to increasefunctionality, such as through increased electrical conductivity,increased light absorption, and enhanced photoseparation of chargecarriers.

Processes for making transparent electrodes for solar cells comprise thesteps of: (a) arranging nanofibers to provide a substantially parallelnanofiber array having a degree of inter-fiber connectivity within thenanofiber array; and (b) drawing said nanofibers from the nanofiberarray as a ribbon or sheet without substantially twisting the ribbon orsheet, wherein the ribbon or sheet is at least about one millimeter inwidth.

In some embodiments, the present invention is directed to applicationsof transparent nanofiber sheets and ribbons for charge collection andcharge recombination layers in organic (also referred to as plastic,excitonic, or hybrid) solar cells and photodetectors and tandem (i.e.,multijunction) solar cells, and also in inorganic thin film solar cellssuch as solar cells based on CuInSe₂, CdTe, GaAs, or GaP.

Charge collecting electrodes of the present invention are optionallypreferably anodes (i.e., hole collectors). These nanofiber sheetelectrodes have many advantages, compared to prior-art transparentconducting oxide electrodes: the nanofiber sheets are flexible,mechanically strong, and tough; they can have anisotropy of optical andelectrical properties (or this anisotropy can be tuned or eliminated bysheet plying), and they have a porous network structure that isespecially favorable for collecting, charge in bulk heterojunction solarcells. Moreover, CNT transparent electrodes have large work functions(5.1-5.3 eV), which is larger than the work function of ITO (4.7 eV) andis favorable for collection of plus charges or holes.

In some embodiments, the present invention provides methods of makingsaid transparent carbon nanotube sheets and ribbons into flexibletextiles as parts of larger solar cell structure. This can be done, forinstance, by including the step of filling the pores of a nanofiberelectrode sheet with conductive polymer or other semiconducting polymer.Importantly, invention embodiments for transparent nanofiber sheet andribbon electrodes are applicable for diverse types of electronicallyconducting nanofibers and nanoribbons, such as those described inSection 2.

In many embodiments, the nanotube sheets comprise carbon nanotubes. Suchcarbon nanotube sheets of the present invention provide uniqueproperties and property combinations such as high optical transparencyin a very broad spectral range, from UV to infrared (300 nm-10 μm), ahigh degree of alignment, toughness, high electrical and thermalconductivities, high absorption of microwave energy in certain frequencydomains, substantial retention of strength even when heated in air at450° C. for one hour, and very high radiation and UV resistance, evenwhen irradiated in air. Furthermore, these nanotube sheets can be drawnwith varying thickness and widths so as to increase their linear density(i.e., the weight per yarn length)

In some embodiments, the fibers or ribbons are chemically and/orphysically modified before or after the draw process or spinningprocess. In some embodiments, the nanofiber yarns are used to formcomposites with other materials useful for solar cells operation, suchas hole transporting and electron transporting molecules and polymers.

(y) Doping of CNT Electrodes: Electrical Conductivity Tuning

It is well known that doping of SWNTs, with either donors or acceptors,shifts the Fermi energy (E_(F)) in the electronic band structure and canchange the electronic conduction mechanism of SWNTs. Vapor exposure withalkaline metals and electrochemical doping using liquid electrolyte alsoprovides important increases of electrical conductivity for the therebydoped CNTs.

Herein, Applicants describe improvement of the transparent electrodeconductivity and match the work function of charge-collecting CNTs withthose of impregnated polymer or monomer structures by creating a doublelayer structure on highly developed CNT surfaces.

Applicants have demonstrated that an extremely high surface area of CNTsheets and ribbons allows one to obtain superior capacitance and densityof charge injection. The electrochemical capacitance can exceed 100 F/g,and nanofiber, sheets, and yarns having such large capacitance areoptionally preferred. This large electrochemical capacitance enablestuning the electrical conductivity of CNT in a very broad range. CNTsheets charged in liquid electrolytes retains double layer ions on thenanotube surface—even when removed from electrolyte, washed in deionizedwater, and dried in vacuum as described in detail in PCT/US2005/007084,filed Mar. 4, 2005.

Because aqueous electrochemistry is limited by a narrow potentialwindow, Applicants use non-aqueous electrochemistry for heavyelectrochemical doping. One good candidate is the electrolyte solutionof LiClO₄ in acetonitrile. Another very promising candidate is1-methyl-3-butylimidazolium tetrafluoroborate, which has an extremelybroad window (−2.4 V to 1.7 V vs. Fc/Fc⁺) of electrochemical stability(L. Kavan, L. Dunsch, Chem. Phys. Chem. 4, 9, 944-950 (2003)).

Applicants describe herein a procedure comprising forming a transparentCNT sheet, doping of said sheet by charge injection in liquidelectrolyte, removal of the CNT sheet from the liquid electrolyte, andintegration of said doped sheet with polymer composite for use in asolar cell. In the first step, a thin transparent CNT film with athickness of optionally preferably less than 200 nm is deposited on anysubstrate as a free-standing sheet. Either SWNTs, MWNTs, or combinationsthereof can be used for this process. In the next step, the CNT sheet iselectrochemically charged from liquid electrolyte to enhanceconductivity of the CNT sheet and tune its work function. Then the CNTsheet can optionally be removed from the electrolyte, dried, anincorporated in a device that is dependent upon the level of chargeinjection, such as a solar cell.

The Applicants have reduced the work function of the CNT electrode bydoping, for example, with alkaline cations. The commonly-assignedco-pending patent application PCT/US2005/007084 on double layer chargeinjection describes in detail the charge injection process into a SWNTmaterial by electrochemical methods. To increase the charge density, anincreased applied potential (measured with respect to the potential ofzero charge injection) is needed. To achieve a high density of injected,the Applicants optionally prefer use of an electrolyte that has highredox stability with respect to either oxidation or reduction (dependingupon the sign of the desired injected charge). The optionally preferredtype of injected charge are holes (positive charge), since this type ofcharge injection generally confers the highest stability for theinjected charge.

Applicants optionally prefer that the electrolyte used for chargeinjection has high stability of electrochemical hole injection. One suchpreferred electrolyte is tetra-n-butylammoniumhexafluorophosphate(TBAPF₆) in acetonitrile.

The following is a typical process of the Applicants inventionembodiments. After electrochemical charge injection in the nanotubesheet or ribbon, the charged electrode is removed from electrolyte andcarefully washed using solvent that is stable with respect to chargeinjected into the nanotube sheet or ribbon (which in some cases can bedeionized water). The nanotube electrode material is dried in an inertatmosphere, wherein such drying can optionally be at an elevatedtemperature (optionally preferably above 50° C.), and then covered by aconjugated polymer thin layer using dip-coating method in two steps (seePCT/US2005/007084). First of all, a very thin (20-30 nm)electron-blocking layer (such as PEDOT-PSS) is dip-coated from a verydilute aqueous solution. Heat treatment at about 100° C. is desired toremove water. In the next step, a suspension/mixture of regio-regularP3HT polymer with very short SWNTs (preferably semiconducting) and C₆₀powder in the ratio 3:1 is prepared by dip-coating on the chargeinjected transparent CNT electrode. The concentration of shortened SWNTin polymer should be below the percolation threshold. In the last stage,the heat treated “sandwich” is covered with an Al electrode andencapsulated.

In an alternative procedure, the deposited CNT thin film can be coveredwith PEDOT-PSS from a liquid phase. Subsequent steps can vary dependingon electrolyte, sign of the injected charge, and the polymer types.

As an alternative to electrochemical charge injection, charge injectionto modify the conductivity and work function of nanotube sheetelectrodes (and other useful nanofiber electrodes) can be by chemicalprocesses by using either electron donating or electron acceptingagents. Such methods are well known in the art, and are widely used forgraphite, carbon nanotubes, and conjugated polymers.

(z) Electrochemical Doping of CNT Electrodes: Work Function Tuning

Both chemical and electrochemical doping of carbon nanotubes is a veryattractive method for tuning the Fermi level by changing the populationof electronic states. For example, it has been shown that semiconductingCNTs can be doped amphoterically; p- and n-type. Optical and electricalmeasurements have confirmed that upon p-type doping, the Fermi level canbe moved down by depleting the filled valence-bands or moved up byfilling empty conduction bands. The concentration of charge carriersincreases drastically when the Fermi level reaches valence andconduction band van Hove singularities. The Applicants show themodulation of field emission I-V curves of SWNT films upon doping, whichdemonstrates the tuning of the work function. A downshift of the Fermilevel (E_(F)) by about 0.4 eV has been demonstrated earlier forelectrochemical doping MWNTs. Larger negative work function shifts ofca. −1.0 eV have also been observed (see PCT/US2005/007084).

(aa) Nitrogen and Boron Doped Carbon Nanotubes for Transparent NanofiberElectrodes

Recently, B- and N-doping within the graphene layers of both SWNTs andMWNTs has attracted great interest, since the electronic properties ofC—N_(x) and C—B_(x) tubes have been found to be quite sensitive to smallamounts of intralayer dopants: nitrogen as the n-dopant and boron as thep-dopant in C-planes. Summarizing the basic results, B and N are wellknown to influence not only properties but, via the synthesis, also thestructure of the doped nanotubes. So, N involved in chemical vapordeposition (CVD) synthesis creates bamboo-type tubes, B in CVD synthesismakes longer tubes (being concentrated at the tips of tubes, B inhibitsthe closing of tubes). The capped walls have also been observed, thusmaking interesting topologies possible where walls have smooth cappedendings. It has already been shown by computations, and partly byexperiment, that during the synthesis of MWNTs, the B atoms not onlyinhibit closing of tube caps and thus strongly enhance the length, butalso make favorable the growth of zig-zag type tubes of specificchirality, all of them being metallic.

Combined doping, electrochemical or chemical non-covalent dopingcombined with intralayer doping, can be usefully applied. Applicantshave demonstrated that charge injection into tubes by chemical orelectrochemical doping is an important process that can change theproperties of tubes: i.e., conductivity, electron emission properties(via work function modulation), optical and IR reflectivity, thermalproperties, etc. Most importantly, the electrochemically injected chargeis stable even in the absence of an electrolyte.

The utility of combined doping is illustrated by the example ofnanotubes doped intra-sheet with nitrogen to produce C—N_(y) nanotubes.C—N_(x) nanotubes have electronic defect levels inside the gap, socharge injected chemically or electrochemically into such preformedC—N_(x) tubes by non-covalent processes will have increased stability.

Even less than 1% molar intrasheet doping of SWNTs or MWNTs with N, B,or combinations thereof can be usefully applied for modifying workfunction, electron emission, electrical conductivity, and thermoelectricpower for the nanotube sheets, ribbons, and yarns of inventionembodiments. The level of intrasheet doping with N, B, or combinationsis optionally preferably above 0.5% for the nanotube sheets, ribbons,and yarns of invention embodiments. This intra-level doping ispreferably achieved before fabrication of the nanotube sheets, ribbons,and yarns. However, it is also useful to fabricate nanotube sheets,ribbons, and yarns from preformed nanotubes, and then add otheroptionally N or B doped nanotubes to these articles by a secondarysynthesis or infiltration process.

(bb) Three-Dimensional Electronic Textiles of Nanofibers and NanofiberYarns Incorporated into Nanofiber Sheets and Ribbons

FIGS. 95 and 96 illustrate nanofiber sheet or ribbon electrodes in whichnanofibers and other electronically functional materials areincorporated in the pore volume of the nanofiber sheets or ribbons inorder to enhance photocharge generation capability.

FIG. 95 illustrates schematically a solar cell, based on nanoscaleintegration of a conducting polymer (CP) and C₆₀ in a carbon nanotubesheet to provide a photoactive donor-acceptor heterojunction within aporous transparent CNT anode (hole collector). The CP/CNT wires arewithin the pores of the CNT sheet, and the intra-pore nanofibers serveas photo electron collectors.

FIG. 96 indicates more reliably the relative dimensions for the porevolume in the nanotube sheet and intra-pore elements. The insets showthe hole transfer from CP chains into the transparent CNT sheet anode,and electron transfer, upon exciton dissociation in the intra-poreCP/CNT system. Holes photogenerated within the approximately 100 nmdiameter pores will be collected on the MWNTs of the sheet, since thecharge collection length of the polymer is about 100 nm.

Semiconducting nanofibers, such as chiral single wall carbon nanotubesor nanofibers of WS₂, MoSe₂, and the like, can be similarly used forformation of intrasheet heterojunctions that enhance charge collectionefficiency.

(cc) Photoelectrochemical Cell with Transparent Nanofiber Sheet and YarnElectrodes as Charge Collectors and Recombination/Separation Layers inTandem Cells

In some embodiments, the present invention relates to specific types ofelectrodes for photoelectrochemical cells, and more precisely for dye-or quantum dot-sensitized solar cells. Some such invention embodimentsalso relate to specific types of conductive electrodes fordye-sensitized and quantum dot-sensitized solar cells: hole collectingcounter-electrodes and transparent separation layers, in multijunctionsolar cells, also known as tandem cells. Methods, processes andarchitectures are described for application of transparent carbonnanotube and other nanofiber sheets, ribbons and yarns both as chargecollector transparent electrodes in dye-sensitized and quantumdot-sensitized electrochemical solar cells, and as transparentseparation layers (also called charge recombination layers) inmultijunction (also called as tandem) dye solar cells. Additionalfunctionality of carbon nanotube charge collectors for enhancement oflight absorption and charge generation due to a nano-antenna effect insolar cells, and other advantages are also described herein.

In some embodiments, the present invention relates to a regenerativephotoelectrochemical cell (PEC) and, more particularly, to a cell ofthis type that uses transparent, mechanically strong, flexible nanofiberyarns, ribbons, or sheets. A PEC is a electrochemical device which, uponabsorption of light, generates charge carriers and, as a result of suchphotogeneration, creates an electrical voltage (potential difference)between its two electrodes: cathode and anode.

One representative type of PEC is a dye-sensitized solar cell (DSC),first developed by Michael Graetzel and colleagues in Switzerland atEPFL in 1991 (see U.S. Pat. Nos. 5,350,644; 5,441,827; and 5,728,487,and Nature 353, 737 (1991), and Nature 395 583 (1998)). DSCs are analogsof photosynthetic natural systems and therefore are intrinsicallyenvironmentally-friendly and are very attractive since their productioncost is relatively low compared to silicon-based and other inorganicsemiconductor p/n junction solar cells.

DSC has achieved a high certified conversion efficiency of 10.4%.Although providing a high efficiency, this cell nonetheless has severaldisadvantages. Present DSCs cannot be made lightweight and flexible.There are, however, numerous applications for which it would bepreferable or even essential that the cell is solid-state,mechanically-strong and at the same time, light-weight and flexible. Themajor problem in the development of lightweight flexible DSCs isreplacement of glass substrates. Glass substrates are fragile, heavy,not very impact-resistant, and have form and shape limitations.

Counter electrode (reduction electrodes) in DSCs are usually constructedof expensive transparent conducting glass substrates coated with Ptcatalyst films (about 60% of the total cost of DSCs). Therefore, inorder to reduce the cost of DSCs and broaden their applications, it isnecessary to develop transparent, flexible, and mechanically-strongelectrodes on substrates other than glass. Efficiency can be increasedin tandem DSCs by using multiple junction architectures. The firsttandem DSC comprising two compartment cells connected in parallel hasbeen demonstrated recently (see M. Dürr et al., Appl. Phys. Lett. 84,3397 (2004) and W. Kubo et al., J. Photochem. Photobiology A 164, 33-39(2004)).

In some embodiments, the present invention is directed to application oftransparent carbon nanotube sheets and ribbons as charge collection andcharge recombination layers in photoelectrochemical (also calledGraetzel or dye-sensitized) solar cells and tandem (i.e.,multi-junction) versions of such solar cells. Charge collectingelectrodes of some invention embodiments are preferably anodes, i.e.,hole or positive charge collectors, which have many advantages over theprior art: they are flexible, mechanically strong, have anisotropy ofoptical and electrical properties, and have a structure ofthree-dimensional networks, especially favorable for collecting chargein bulk heterojunction solar cells. Moreover, CNT transparent electrodeshave large work functions (5.1-5.3 eV), which is larger than the workfunction of ITO (4.7 eV), which is favorable for collection of positivecharge or holes.

FIG. 100 is a schematic diagram showing the basic structure of theprior-art dye-sensitized solar cell. The DSC comprises: (1) a firsttransparent substrate having respective transparent electrode 10001, (2)a second non-transparent conducting electrode 10002, preferablyreflective (for simplicity, electrodes 10001 and 10002 are designatedfirst electrode and second electrode), (3) the first electrode 10001 hasa photoelectrochemically active semiconductor oxide made in the form ofa porous nanostructure formed of sintered colloidal particles 10003, and(4) the photo-electrochemically active semiconductor oxide is coatedwith a monolayer of dye molecules or quantum dots 10004. Both of saidelectrodes are arranged in such manner that electrode 10003 and 10002face each other, the sides are sealed with rubber, resin, or the like,and the space between 10003 and 10002 is filled with an electrolyte10005 which comprises a redox couple in a conventional manner, and whichimpregnates the porous structure of semiconductor oxide 10003 in suchmanner that the interface between electrolyte 10005 and dye- orquantum-dot-coated oxide has a very large effective interface. In someembodiments of the present invention, the transparent substrate ofelectrode 10001 is covered with a transparent nanofiber sheet or ribbon.

Invention embodiments described herein provide novel architectures,processing and fabrication methods, compositions of matter, andapplications of transparent nanofiber sheets having properties andfunctionalities useful for photoelectrochemical solar cells design. Forexample, carbon nanotube sheets of invention embodiments provide thefollowing unique properties and unique property combinations,particularly useful for photoelectrochemical solar cells: (1) highoptical transparency (80-90%); (2) low electrical sheet resistance, (3)high thermal conductivities and thermal diffusivities, (4) high workfunction required for collection of plus charges, holes in solar cells,(5) high flexibility opposed to brittle ITO and other inorganic TCOs,(6) very high resistance to creep, (7) three dimensional morphology,which is favorable for collection of charge carriers from the bulkheterojunction types of architectures, (8) high electrochemical activityin terms of efficient and fast charge transfer between electrolyte andnanofibers due to high surface area and matched energetics, and (9) veryhigh radiation and UV resistance—even when irradiated an electrolyte.

Such a new photoelectrochemical cell as described above, for example, isshown in FIG. 101. FIG. 101 is a schematic view of an embodiment of aphotoelectrochemical cell of the present invention. Thephotoelectrochemical cell includes a wide bandgap semiconductorelectrode having a transparent porous nanofiber electrode 10102, whereinsaid nanofiber electrode is impregnated within its inter-fiber poreswith an active material 10103 selected from the group consisting oftitanium oxide, zinc oxide, tungsten oxide, and mixtures thereof, areduction electrode having a transparent porous nanofiber electrode10105 on glass or plastic substrate, and an oxidation-reductionelectrolyte 10104.

As a transparent substrate 10101, for example, a glass substrate orplastic substrate such as polyethylene naphthalate (PEN) or polyethyleneteraphthalate (PET) can be used. As a transparent electrode 10102,formed on the surface of the transparent substrate 10101, a transparentporous nanofiber sheet electrode is formed instead of a conventionaltransparent conducting oxide such as indium-doped tin oxide orfluorine-doped tin oxide or fluorine-doped indium oxide. The thicknessof the transparent electrode is preferably in the range of 50 to 200 nm.The solar light transmittance of the transparent electrode 10102 ispreferably not less than 50%.

A semiconductor photoelectrode 10103 containing the metal. oxidenanoparticles (for example, 10-20 nm TiO₂ nanoparticles) can be coatedon the surface of the transparent porous nanofiber electrode 10102 by aprinting method or sol-gel method to form a film of approximately 10-20μm thickness. Rapid sintering and anatase phase formation can beperformed by a multi-mode microwave heating at a frequency chosen fromthe range varying from 2 to 30 GHz and power of about 1 kW for 5minutes. After sintering, the layer exhibits a porosity of 0.5-0.65. Aspecific surface area of the highly porous semiconductor electrode is inthe preferable range from 20 to 300 m²/g. The average pore diameter ofthe semiconductor film is preferably in the range of preferably 5 to 250nm. If the average pore diameter is less than about 10 nm or higher thanabout 250 nm, the adsorbed amount of optionally used quantum dotsensitizer is lower than that required for high photoelectric conversionefficiency.

In one embodiment, a monolayer of red dye molecules is attached to thesurface of the highly porous nanofiber-metal oxide electrode byimpregnation, for example, with an absolute ethanol solution of theruthenium dye-II cis-dithiocyanato-N,N-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)-(H₂)(TBA)₂RuL₂(NCS)₂(H₂O)₄sensitizer at a concentration of 20 mg of dye per 100 ml of solution.The impregnation process can be optionally done at room temperatureovernight. The electrode was rinsed with ethanol and then dried. Thecoating solution can be applied by a number of methods, such as dipping,spin-coating, spraying, ink jet printing and screen printing. Thecoating step may be repeated, as necessary.

In another embodiment a similar dipping step can be applied so as toadsorb quantum dot sensitizers such as PbS and PbSe.

In contrast to conventional photoelectrochemical cell design, thedispersion of a binder component with metal oxide particles is notnecessary. The mechanically strong 3-dimensional nature of thetransparent nanofiber sheet electrode of the present invention providesa supporting grid for porous metal oxide structure, taking into accounta high adhesion of metal oxides to nanofibers (K.-H. Jung, J. S. Hong,R. Vittal, K.-J. Kim, Chemistry Letters 864-865 (2002)).

In one embodiment of this invention, the transparent porous nanofiberreduction electrode 10105 in FIG. 101 is fabricated in accordance withmethods described herein. No platinization of counter-electrode isneeded because such nanofibers can operate both as a charge collectingelectrode and as a catalyst for enhancing the electrochemical chargetransfer processes. Catalytic activity of the reduction electrode 10105can be improved by coating the surface of electrode 10105 with a thinlayer of single-wall carbon nanotubes, as shown by SEM image in FIG.102. Both of said electrodes being arranged in such manner, shown inFIG. 101, such that electrodes 10103 and 10102 face each other, thesides are sealed with rubber, resin, or the like.

The space between the transparent photoactive front electrode and thereduction back electrode is filled with an electrolyte which comprises aredox couple in a conventional manner, and which impregnates the porousstructure of semiconductor electrode in such manner that the interfacebetween electrolyte and dye- or QD-coated oxide has a very largeeffective interface. The spacer particles may be interposed between thefront and the back electrodes in order to prevent electrical shorting.

The MWNT nanofiber sheets possess another unique physical property. TheMWNT sheets strongly absorb microwave radiation, as evidenced by theiruse for this welding of plastic parts in a microwave oven: two5-mm-thick plexiglass plates were welded together using the heating of asandwiched MWNT sheet to provide a strong, uniform, and highlytransparent interface in which nanotube orientation and electricalconductivity are maintained. The microwave heating was in a 1.2 kilowattmicrowave oven that operates at 2.45 GHz. Thus, microwave processingoffer an attractive and very promising method of selective heating ofcomposite films. This technique was earlier applied to the preparationof nanosize TiO₂ powder with a high degree of crystallinity andmonodispersed particle sizes (C. Feldman and H. O. Jungk, Angew. Chem.Int. Ed. 359 (2001) and T. Yamamoto et al., Chem. Lett. 964 (2002))

In one embodiment of the present invention, involving preparation ofnanofiber electrochemical electrodes, Applicants have used a microwavesintering method, wherein a carbon nanotube-metal oxide compositephotoelectrode can be prepared by new low-temperature sintering methodsof highly porous and conducting metal oxide film by using microwaveirradiation. The metal oxide (such as TiO₂, WO₃ and ZnO) exhibitsmoderate coupling to microwaves because of low electrical conductivityand low magnetic induction losses. However, carbon nanotubes absorbmicrowaves very efficiently and can be heated very rapidly. Such highlyefficient CNT-microwave coupling correlates with high conductivity andthe nanosize of the carbon nanotubes leads to high induction losses. Thetemperature can reach 1000° C. after microwave irradiation withinminutes or less.

Keeping in mind that the generalized energy loss equation is written asfollows:

P=2πf∈ _(o)∈_(r) tan σE ² V _(s)Θ,

where f and ∈_(o) are the frequency and dielectric constant; tan σ isthe dielectric and magnetic loss factor, E, V_(s), Θ are the electricfield values inside the sample, volume factor, and shape factor,respectively) the absorption of microwaves occurs very effectively.Therefore the microwave heating of a MWNT sheet within a metal oxidematrix provides a strong, uniform, and transparent interface in whichnanotube orientation and sheet electrical conductivity is changedrelatively little.

A counter (reduction) electrode is usually constructed of expensivetransparent conducting glass substrates coated with Pt catalyst films(about 60% of the total cost of DSC) which are fragile, heavy and notimpact resistant and have form and shape limitations. Therefore, inorder to simplify the manufacturing process and reduce the cost of DSCsand broaden their applications without significant deterioration oftheir performance, the transparent photoelectrode can be manufacturedusing transparent or non-transparent nanofiber sheets, ribbons, oryarns.

The following examples are presented to more particularly illustrate theinvention, and should not be construed as limiting the scope of theinvention.

Example 1

This example describes a typical method for growing nanotube forestsused for selected invention embodiments. Aligned MWNT arrays (nanotubeforests) were synthesized by atmospheric pressure CVD (Chemical VaporDeposition) in a 45 mm diameter quartz tube using 5 molar percent C₂H₂in He at 680° C., at a total flow rate of 580 sccm for 10 minutes. Thecatalyst was a 5 nm thick iron film that was deposited on a Si wafersubstrate or glass substrate by electron beam evaporation. Based on SEMand thermal gravimetric measurements, the purity of the spun yarns wasvery high (˜96-98% C in the form of MWNTs), with 2-4% Fe and amorphouscarbon and no observed carbon particles.

Example 2

This example describes a method of invention embodiments for theintroduction of twist during the spinning of carbon nanotubes from ananotube forest. Previous attempts to draw yarns from nanotube forestsresulted in extremely weak assemblies (see K. Jiang, Q. Li, and S. Fanin Nature 419, 801 (2002) and in U.S. Patent Application Publication No.20040053780 (Mar. 18, 2004)). The present inventors find thatsimultaneously applied draw and twist processes increase the mechanicalstrength by a factor of over a thousand. The present inventors also showthat the present spin-twist process is able to produce yarns evensmaller than one micron in diameter. The fibers of this example werehand drawn during twisting by attaching nanotubes from the side of thenanotube forest of Example 1 to a probe tip that is coaxially attachedto the axis of a variable speed motor that was typically operated atabout 2000 rpm. In this example, illustrated by the photograph in FIG.1, attachment was achieved by wrapping a yarn drawn from the nanotubeforest about a miniature wooden spindle, which was attached to thecenter of a motor shaft. The fibers were simultaneously twisted anddrawn by pulling this rotating spindle and attached nanotube yarn awayfrom the nanotube forest. The motor was mounted on a platform (FIG. 1)and the platform was moved by hand along a tabletop to accomplish thedrawing while the motor operated at approximately 2000 rpm to providetwisting. FIG. 2 is a scanning electron microscope (SEM) picture showingnanotube assembly into yarn during the spinning process of the presentinventors, in which the nanotubes are simultaneously drawn from thenanotube forest and twisted. The direction of drawing was orthogonal tothe original nanotube direction and parallel to the plane of thesubstrate, although the spinning process is sufficiently robust that theangle between the nanotube direction in the forest, perpendicular to thesubstrate, and the draw direction can be decreased from 90° to almost00. The obtained combination of yarn diameters that are hundreds oftimes smaller than the nanotube length (˜300 μm) and nanotube twistresulted in yarns having the attractive properties that will bedescribed in other examples.

Example 3

This example shows that extremely small diameter yarns can be spun usingthe method of Example 2. The yarn diameter was determined by controllingthe width of the MWNT forest array that was pulled to generate theinitial wedge-shaped untwisted ribbon that converged from a thickness ofabout the height of the forest to the width of the yarn at the wedgeapex (FIG. 2). The array width ranged from below 150 μm to about 3 mm toproduce singles yarn diameters of between about one and ten microns. A200 μm wide forest array segment produced about a 2 μm diameter twistedyarn and a forest area of 1 cm² could generate an estimated 50 m of thisyarn. The inserted twist was typically about 80,000 turn/m, versus about1000 turns/m for a conventional weaving textile yarn having 80 timeslarger diameter. The twist resulted in densities for the MWNT yarns ofabout 0.8 g/cm³, based on direct measurements of yarn mass, length, andyarn diameter, the latter being measured by SEM. The linear density ofthe singles yarn was typically about 10 μg/m, compared with values of 10mg/m and 20-100 mg/m for cotton and wool yarns, respectively. About ahundred thousand individual nanofibers pass through the cross-section ofa 5 μm diameter nanotube yarn, as compared with the 40-100 fibers in thecross-section of typical commercial wool (worsted) and cotton yarns.

Example 4

This example shows that the twisted fibers of Example 2 have themechanical robustness needed for plying, and that such plying canimprove the mechanical strength of the yarns. FIGS. 3 A, B, and C showSEM images of singles, twofold and fourfold MWNT yarns, respectively.The twofold yarns were obtained by over-twisting a singles yarn andsubsequently allowing it to twist relax around itself until it reached atorque balanced state. The alignment of the individual MWNTs along theaxis of the twofold yarn visually confirms that the twofold structurewas torque balanced. This procedure was repeated for the twofold yarn(using an opposite twist direction) to produce fourfold yarns. FIG. 58is a SEM micrograph showing about twenty MWNT singles yarns that havebeen plied together to make a yarn having a diameter about equal to ahuman hair.

Example 5

This example (using twisted yarns and plied yarns prepared as inExamples 2 and 4) describes twist retention in knitted singles andmultiple ply yarns of carbon nanotube fibers. Unlike singles yarns ofconventional textiles, highly twisted MWNT singles yarns largely retaintwist when the yarn ends are released (FIG. 4, top). This enhancedlocking of twist likely reflects the stronger interactions betweennanotubes than between the microscopic fibers in such textiles as cottonand wool. Particularly surprising, twist is retained up to the locationof the break point for singles and twofold yarns that have been brokenby tensile extension (FIG. 4).

Example 6

This example (using twisted yarns and plied yarns prepared as inExamples 2 and 4) shows that a draw-twist method of inventionembodiments causes a thousand-fold increase in mechanical strengthcompared with prior-art results for nanotube yarns spun from nanotubeforests. While the untwisted yarns were so weak that they broke whenpulled away from surfaces that they accidentally contacted, the singlesyarns had measured tensile strengths between 150 and 300 MPa. Possiblyreflecting the above mentioned increase in nanofiber orientation withrespect to the yarn axis as a result of folding (i.e., plying), higherstrengths of between 250 and 460 MPa were observed for twofold MWNTyarns. Typical stress-strain curves for these singles and twofold yarnsare shown in FIG. 5, wherein the curves correspond to (a) carbon MWNTsingles yarn, (b) twofold yarn, and (c) PVA-infiltrated singles yarn.The stresses on the y-axis in this figure are engineering stresses,based on cross-sectional area measured by SEM for the unstressed yarn.As a result of a giant Poisson's ratio effect (described below inExample 14), the true stress at close to break (normalized to the truecross-sectional area at close to break) is about 30% larger. For spaceand aerospace applications, density-normalized strength is important.Using the maximum observed density of 0.8 g/cm³, the density-normalizedfailure stress of the twofold yarns is between 310 and 575 MPa/gcm⁻³.

Example 7

This example shows that the singles and plied nanotube yarns of Examples2, 3 and 4 have toughness comparable to that of high performancepolymers used for antiballistic vests. The pure nanotube yarns had amuch larger strain-to-failure (up to 13%) than for graphite fibers(˜1%). This high failure strain, combined with high failure strength,meant that the work needed to break the yarns (called toughness) wasalso high: about 14 J/g for the singles yarn, 20 J/g for the twofoldyarn and 11 J/g for the PVA infiltrated singles yarns, which combinedtheir higher strength with a lower strain-to-failure (about 3-4%). Whilethe toughness for the twofold yarn (20 J/g) is above that of graphitefibers (12 J/g) and approaches that of commercial fibers used forantiballistic vests (˜33 J/g for Kevlar® fibers), far greater toughnesshas been demonstrated for solution spun SWNT/PVA composite fibers (600J/g). However, this latter energy absorption is largely due toirreversible plastic deformation over large strains, so largedeformations are needed and such large energy absorption can occur onlyonce.

Example 8

This example shows that the carbon nanotube yarns can be easily knitted(see FIG. 3E) and tied into tight knots (FIG. 6). While abrasion andknotting, especially with an overhand knot, seriously degrades thestrength of most polymer fibers and yarns (including the Kevlar® andSpectra® fibers and yarns used for antiballistic vests, conventionaltextile yarns, and even single polymer chains—causing rupture at theentrance to the knot) this is not the case for the investigated singlesand twofold nanotube yarns, where tensile failure has only been observedfar from an inserted overhand knot. High abrasion resistance issuggested by the absence of ultimate tensile failure in a long yarn loopthat was pulled through a very tight overhand knot.

Example 9

This example, for comparison with Example 2, 37, and 52, shows thattwisting nanotube sheet strip (cut parallel to the orientationdirection) does not provide desired mechanical properties enhancementsunless the processes and nanotube characteristics described in theinvention embodiments are utilized. The oriented films from which thesheet strips were cut were magnetically oriented assemblies of micronlength nanotubes (see J. E. Fischer et al., J. Applied Phys. 93, 2157(2003)). Also, the inventors were not able to pull continuous nanotubeyarns from the sides or tops of these magnetically oriented nanotubesheets or sheet strips by using draw directions that result is drawableand twistable yarns for the much longer nanotubes of inventionembodiments.

Example 10

This example shows that the mechanical properties of twisted yarns canbe increased by infiltration of a polymer, polyvinyl alcohol (PVA). Theyarns utilized were made by the process of Example 2. MWNT/PVA compositeyarns were made either by soaking a singles yarn for 15 hours in 5 wt %aqueous PVA solution or by passing a singles yarn through a drop of thissolution during spinning, and then drying. The molecular weight of thePVA was in the range 77000-79000 and it was 99.0-99.8% hydrolyzed.Infiltration with PVA increased the observed strengths of singles yarnsto 850 MPa. A typical stress-strain curve for the PVA-infiltratednanotube yarn of this example is shown in FIG. 5 c. FIG. 57 provides SEMmicrographs showing that the PVA infiltration has not disrupted thetwist-based structure of the MWNT yarn.

Example 11

This example shows that the twisted nanotube-based yarns have highelectrical conductivity both before and after the twisted nanotube yarnis converted into a nanotube/poly(vinyl alcohol) composite yarn (usingthe method of Example 10). The investigated yarns (diameters from 2 μmto 10 μm) had a four-probe electrical conductivity at room temperatureof about 300 S/cm and a negative temperature dependence on resistance(about −0.1% per ° K between 77 and 300 K). PVA infiltration decreasedthe electrical conductivity of the yarns by only about 30%, leading tonanotube/PVA composite yarns that have over 150 times higher electricalconductivity than observed for nanotube composite fibers containinginsulating polymers.

Example 12

This example shows that (a) the twisted carbon nanotube yarns have adramatically increased elastic strain region compared with that of theprior-art, high strength carbon nanotube yarns, (b) this long elasticregion provides a high elastically recoverable component to yarntoughness, and (c) the reversible deformation of the yarn is hysteretic.The nanotube yarns show hysteretic stress-strain curves when subjectedto load-unload cycles (FIG. 7). While complete unloading from theinitial load does not return the yarn to its original length, theinitial hysteresis loop is essentially unshifted on subsequent cycling.Depending upon the initial strain, the observed energy loss perstress-strain cycle of a twofold MWNT yarn is in the 9%-22% range for0.5% cycle strain, the 24%-28% range for 1.5% cycle strain (FIG. 8), andthe 39%-48% range for the maximum reversible cycle strain (2%-3% fortotal strains up to 8%). Within a hysteresis loop, the effective moduluson initial unloading and initial reloading is much larger than for thefinal parts of the unloading and reloading steps (FIG. 9). Also relevantfor applications, the failure strength of nanotube yarn (singles anddoubles) was unaffected by 50 loading-unloading cycles over a stressrange of 50% of the failure stress. Unlike nanotube sheets made byfiltration of nanotube solutions, the nanotube yarns are resistant tocreep and associated stress relaxation—the stress relaxed no more than15% when a twofold nanotube yarn was held for 20 hours at 6% strain (170MPa initial stress), and this small stress relaxation occurred withinthe first 20 minutes and was largely viscoelastic (i.e., reversible).

Example 13

This example demonstrates the extreme stability of the mechanicalproperties of twisted nanotube yarns in air at high temperatures, aswell as retention of properties at cryogenic temperatures. These yarnswere made by the method of Example 2. The failure strength of a twofoldyarn (300 MPa) was essentially unchanged after heating in air at 450° C.for an hour. Although evidence of air-oxidation was evident in SEMmicrographs, a nanotube yarn held at 450° C. for 10 hours wassufficiently strong and flexible to be tightly knotted. Tight knot tyingwas also possible while the nanotube yarn was immersed in liquidnitrogen.

Example 14

This example demonstrates that the inventors have achieved giantPoisson's ratios for the carbon nanotube yarns spun by the process ofExample 2. They observe giant Poisson's ratios for the nanotube yarns,which increases with increasing strain from 2.0 to 2.7 for MWNT singlesyarn and from 3.3 to 4.2 for twofold yarn (FIG. 10). These Poisson'sratios, measured while stretching the yarn in a SEM, are up to 12 timeslarger than observed in orthogonal directions for ordinary solids. APoisson's ratio of 4.2 means that elongating the yarn by a strain Eprovides a 4.2 times larger strain in each of the lateral dimensions anda fractional volume decrease of 7.4 ∈, versus the fractional volumeincrease of about 0.4 s for ordinary solids. Hence, the nanotube yarnsare stretch-densified, which is quite a rare property for solids. Out of500 crystal phases investigated in a global search of the literature,only 13 were found to be stretch densified (see R. H. Baughman, S.Stafström, C. Cui, S. O. Dantas, Science 279, 1522 (1998)). Astretch-densified material must have a negative linear compressibility,meaning in the present case that the yarn length increases when the yarnis hydrostatically compressed with a non-penetrating hydrostatic media.These giant Poisson's ratios and the associated stretch-induced volumedecrease of up to 7.4 ∈ might be used for tuning the absorption andpermeability of nanotube yarns and textiles by applying small appliedstrains in the yarn direction (or directions). The inventors abovedescribe the application of this stretch densification for thefabrication of electronic devices (using their novel knottronicsapproach for obtaining the patterning capabilities that are ordinarilyobtained for electronic devices using photo lithography).

Example 15

This example demonstrates the application of a nanotube yarn spun by thetwist-draw process (Example 2) and plied (Example 4) for the filament ofan incandescent light. FIG. 18 is a picture of a twofold twistedmultiwalled nanotube yarn that has been electrically heated toincandescence in an inert atmosphere chamber. The yarn is wound betweentwo metal leads that are spaced apart by about 20 mm and silver paste isapplied to the yarn at the junctions with the metal leads to lowerresistance. The filament so formed emits light when a voltage isapplied. While it has been well known from the time of Thomas Edison,this is the first example of the application of a twisted, hightoughness yarn for this application (see Example 7 for the measurementresults for the toughness of the twisted yarns). The existence of thistoughness can enable the fabrication of incandescent light bulbs thatare more resistant to filament failure by mechanical damage than areconventional incandescent light bulbs.

Example 16

This example demonstrates the drawing of carbon nanotubes from amulti-wall carbon nanotube forest to form a transparent nanotube ribbon,as well as the wrapping of this nanotube ribbon about a mandrel (ahollow capillary tube), to produce the transparent article of FIG. 11.Using the nanotube forest of Example 1, the inventors surprisingly foundthat nanotube ribbons having arbitrarily wide widths could be drawn fromthe forest and that these ribbons are optically transparent. The widthof the obtained ribbon essentially equaled the width of the nanotubeforest sidewall that was pulled (without twisting) from the forest.These ribbons were sufficiently mechanically robust that they could beeasily manipulated without breaking. FIG. 11 shows an approximatelymillimeter-width nanotube ribbon that was helically wrapped on amillimeter diameter glass capillary tube. The transparency of the ribbonis indicated by the visibility of the printed line on the sheet of paperthat is beneath the nanotube-ribbon-wrapped glass capillary tube.

Example 17

The method of Example 16 can be used for the fabrication of hollow tubesthat can be employed for cell growth, such as neurons. This capabilityis important, since while the suitability of nanotubes for cell growthis well established in the prior art, methods have not be described forthe fabrication of suitably shaped nanotube assemblies for this purpose.A hollow tube comprising nanotube ribbon can be prepared as in Example16. The choice of mandrel is made to facilitate subsequent removal ofthe wrapped ribbon from the cylindrical mandrel. Sufficient layers ofnanotube ribbon to provide needed mechanical strength can be wrappedeither as a helix or as two oppositely wrapped helices. The mechanicalstrength of the helically wrapped ribbons can be optionally enhancedusing the liquid-based densification process of Example 23. The mandrelwrapped nanotube ribbon can then be removed from the mandrel to providea hollow tube that is suitable as a substrate for cell growth. Thisremoval from the mandrel can be accomplished is various ways. One methodis by using a mandrel that is a polymer that depolymerizes andevaporates at low temperatures. Another method is by coating a glasscapillary tube with an easily solubilized coating, whose dissolutionenables the hollow tube comprising nanotube ribbon to be slipped fromthe mandrel.

Example 18

This example demonstrates fabrication and deposition of a very wide MWNTribbon on a glass substrate to provide a transparent electricallyconducting layer. This very wide ribbon was drawn from the nanotubeforest of Example 1 by pulling a forest sidewall section thatapproximately equals the ribbon width. Details on the various ways thatthe inventors were able to conduct this draw are provided in Examples 21and 46. FIG. 17 pictures the nanotube sheet, after it was mechanicallypressed onto the glass substrate. Because the nanotubes are well alignedin the sheet, the electrical and optical properties are anisotropic. Theprinting is on a sheet of white paper that is beneath the electricallyconducting layer. The visibility of the printing indicates thetransparency of this electrically conducting sheet.

Example 19

This example describes a method for fabricating a twisted yarn that isan electrochemical device, such as a supercapacitor or a battery. Acarbon nanotube singles yarn (Example 2) is over twisted to the amountsufficient to form a twofold yarn (as in Example 4). While held at thetension used for the twist process, the twisted yarn is exposed to anaqueous solution comprising polyvinyl alcohol and phosphoric acid, orother well-known media suitable as an electrolyte. This exposure resultsin imbibing this electrolyte precursor into the yarn and overcoating it.Residual liquid is then optionally removed by evaporation to form asolid electrolyte or a gel electrolyte. The opposite ends of theelectrolyte coated fiber are then brought together to permittwist-relaxation to form a twofold yarn in which the electrolyte coatingprevents direct lateral electrical contact between the two componentsingles yarns that comprise the twofold yarn. If the electrolyte is toomechanically resistant for this plying to automatically occur, the twoyarn segments can be mechanically wound together in another twistprocess. This twofold yarn is then optionally overcoated with additionalelectrolyte. The loop end of the twofold yarn is then cut so that thetwo fibers are then electronically isolated with respect to one another,being in contact only through the electrolyte gel. The twoelectronically separated electrolyte-filled singles yarns within thetwofold yarn can potentially serve as opposite electrodes for a fibersupercapacitor. As an alternative to the above fabrication method, twotwisted singles yarns could be coated and imbibed with electrolyte,optionally partially dried, and then twisted together or otherwisecombined in side-by-side contact prior to an optional step of overcoating the fiber pair with electrolyte. The practice of this lattermethod for untwisted nanotube fibers is described by A. B. Dalton et al.in Nature 423, 703 (2003). Analogous methods can be used to make a fiberbattery. However, in order to achieve high energy storage density forsuch fiber supercapacitor, the electrolyte used for the above processesis optimally selected to have a high redox stability range and the saltcomponent in the electrolyte is preferably selected to be a lithium saltof a type conventionally selected for lithium batteries.

Example 20

This example shows that nanotube yarns can be optionally densified priorto twisting using the surface tension effects of an imbibed liquid.After the nanotube yarn is pulled from the forest, it is then passedthrough a liquid bath or exposed to a liquid vapor. Suitable liquids forsuch densification of yarns pulled from the forest of Example 1 includemethanol, isopropyl alcohol, and acetone. The evaporation of the liquidabsorbed in the yarn causes shrinkage in the lateral direction, leadingto densification. The inventors show in Example 38 that densification ofdrawn ribbons prior to twist made it possible to obtain uniformlytwisted yarn even when the applied twist is very low (corresponding to ahelix angle of 5°). Application of such low twist in the absence ofpre-applied liquid-based yarn densification resulted in non-uniformtwist and yarn diameter.

Example 21

This example shows that a continuous, transparent nanotube sheet havinghigh strength can be drawn from a sidewall of multiwalled nanotube(MWNT) forest of Example 1. The MWNTs were ˜10 nm in diameter and therange of investigated forest heights was 50 to 300 μm. Draw wasinitiated using an adhesive strip to contact MWNTs teased from theforest sidewall. Meter-long sheets, up to 5 cm wide, were then made at ameter/minute by hand drawing (FIG. 21). Sheet transparency isillustrated by the visibility of the NanoTech Institute logo that isbehind the MWNT sheet. Despite a measured areal density of only ˜2.7μg/cm², these 500 cm² sheets are self-supporting during draw. A onecentimeter length of 245 μm high forest converted to about athree-meter-long free-standing MWNT sheet. The sheet production rate wasincreased to 5 m/min by using an automated linear translation stage toaccomplish draw at up to 10 m/min by winding the sheet on a rotatingcm-diameter plastic cylinder. The sheet fabrication process is quiterobust and no fundamental limitations on sheet width and length areapparent: the obtained 5 cm sheet width equaled the forest width whenthe draw rate was about 5 m/min or lower. The nanotubes are highlyaligned in the draw direction, as indicated by the striations in the SEMmicrograph of FIG. 22. This draw process does not work for most types ofMWNT forests and the maximum allowable draw rate depends on thestructure of the forest. Intermittent bundling within the forest isuseful, wherein individual nanotubes migrate from one bundle of a fewnanotubes to another. Bundled nanotubes are simultaneously pulled fromdifferent elevations in the forest sidewall, so that they join withbundled nanotubes that have reached the top and bottom of the forest,thereby minimizing breaks in the resulting fibrils (FIGS. 22 and 23).Disordered regions exist at the top and bottom of the forests, where afraction of the nanotubes form loops, which might help maintaincontinuity. For forests having similar topology and nanotube lengths inthe 50 to 300 micron range for different forests, the longer nanotubes(corresponding to the higher forests) were easiest to draw intosheets—likely because increasing nanotube length increases inter-fibrilmechanical coupling within the sheets.

Example 22

This example shows that the solid state drawn nanotube sheet of Example21 comprises a novel, useful state of matter that was previouslyunknown: an aerogel comprising highly oriented carbon nanotubes. Fromthe measured areal density of about 2.7 μg/cm² and the sheet thicknessof about 18 μm, the volumetric density is approximately 0.0015 g/cm³.Hence, the as-produced sheets are an electronically conducting, highlyanisotropic aerogel that is transparent and strong. The high degree ofnanotube orientation in the nanotube sheet is demonstrated by the Ramanspectra of FIG. 41 of an as-drawn four-sheet stack in which all sheetshave the same orientation. A VV configuration (parallel polarization forincident light and Raman signal) was used, with polarization parallel to(∥) or perpendicular to (⊥) the draw direction of the nanotube sheets.The ratio of Raman intensity (632.8 nm excitation) of the G band forpolarization parallel and perpendicular to the draw direction is between5.5 and 7.0 for the VV configuration (parallel polarization for incidentlight and Raman signal), which correspond to polarization degrees of0.69 and 0.75, respectively, for the investigated four-sheet stacks(FIG. 41). The anisotropy of light absorption (FIG. 25) also indicatedthe high anisotropy of the nanotube sheets. Ignoring the effect of lightscattering, the ratio of absorption coefficient for parallel andperpendicular polarizations for the as-drawn single sheet was 4.1 at 633nm, and monotonically increased to 6.1 at 2.2 m. The striations parallelto the draw direction in the SEM micrograph of FIG. 22 provides moreevidence for the high degree of nanotube orientation for the as-drawnnanotube sheets.

Example 23

This example shows that the inventors can easily densify these highlyanisotropic aerogel sheets into highly oriented, transparent,electrically conducting sheets having a thickness of 30-50 nm and adensity of ˜0.5 g/cm³. The inventors obtain this 360-fold densityincrease by simply adhering by contact the as-produced sheet to a planarsubstrate (e.g., glass, many plastics, silicon, gold, copper, aluminum,and steel), immersing the substrate with attached MWNT sheet into aliquid (e.g. ethanol), retracting the substrate from the liquid, andthen retracting the substrate from the liquid, and then permittingevaporation. Densification of the entire sheet, or selected areas withinthe sheet, can also be similarly obtained by dropping or otherwiseinjecting such a liquid onto the sheet area where densification isdesired, and allowing evaporation. Surface tension effects duringethanol evaporation shrink the aerogel sheet thickness to ˜50 nm for theMWNT sheet prepared as described in Example 1. SEM micrographs takennormal to the sheet plane suggest a small decrease in nanotubeorientation as a result of densification. However, this observation isdeceptive, as the collapse of ˜20 μm sheets to ˜50 nm sheets withoutchanges in lateral sheet dimensions means that out-of-plane deviationsin nanotube orientation become in-plane deviations that are noticeablein the SEM micrographs. The aerogel sheets can be effectively glued to asubstrate by contacting selected regions with ethanol, and allowingevaporation to densify the aerogel sheet. Adhesion increases because thecollapse of aerogel thickness increases contact area between thenanotubes and the substrate. The performance of liquids that do notperform well for liquid-based sheet, ribbon, or yarn densification for aparticular type of nanotube can be enhanced by adding a suitablesurfactant. For example, water does not perform satisfactorily fordensifying the nanotube sheets prepared using the method of Example 21from the nanotube forests of Example 1. However, a surfactant/watermixture (either 0.7 weight percent Triton X-100 in water or 1.2 weightpercent lithium dodecyl sulfonated in water) was a satisfactorydensification agent. Other considerations for the choice of liquid fordensification are liquid viscosity (which affects the rate of the liquidinfiltration process) and the ease at which this liquid can bevolatilized during subsequent processing. Quite surprisingly, the sheetresistance (FIG. 24) in the draw direction changes by <10% upon sheetdensification by a factor of ˜360, which increases sheet transparency(FIG. 25). While the anisotropy ratio for sheet resistance decreasesfrom 50-70 for the undensified sheets to 10-20 for the densified sheets,this anisotropy ratio for the densified sheets is nearly temperatureinvariant.

Example 24

For comparison purposes with the nanotube sheets made by the presenttechnologies, this example shows the application of conventionalfiltration-based processes for the fabrication of SWNT and MWNT sheets.In other examples, the inventors will compare the properties of theseconventionally prepared sheets with those of the present technologies.The forest-grown MWNTs of Example 1 were used for the filtration-basedsheet fabrication process. The latter sheets made by the filtrationroute utilized an ultrasonically dispersed mixture of 0.07 wt % MWNTs inan aqueous solution containing 0.7 wt % Triton® X-100 as surfactant. TheSWNT sheets were analogously made using carbon monoxide synthesizenanotubes (HiPco) obtained from Carbon Nanotechnologies, Inc. Prior tomeasurements, residual surfactant was removed from these MWNT and SWNTsheets by thermal annealing at temperatures up to 1000° C. in argon.Thermal gravimetric analysis of forest-derived MWNTs in oxygen showsthat they contain at most 4% non-combustible weight, which is likely dueto catalyst particles. The wt % catalyst in the HiPco nanotubes is ˜30%.

Example 25

This example shows that both the as-drawn and densified MWNT sheets havea very small temperature dependence of resistivity and low noise powerdensity, which indicates that these MWNT sheets are highly suitable foruse for sensor applications. In fact, the temperature dependence ofsheet resistivity is nearly the same for the forest-drawn densifiednanotube sheets and sheets made by the filtration route using the sameforest-grown MWNTs, and much smaller than for single-walled nanotubesheets fabricated by filtration (FIG. 24). In addition, the lowfrequency (f) noise power density in the draw direction for a densifiedforest-drawn sheet is 10⁴ and 10 times lower than for ordinaryfiltration-produced sheets of SWNTs and MWNTs, respectively (FIG. 26).

Example 26

This example shows the forest-drawn MWNT sheets can be convenientlyassembled into biaxially reinforced sheet arrays. These sheets wereprepared using the method of Example 21. A four ply biaxially reinforcedsheet array is shown in FIG. 27. Chiral structures, which will likely beoptically active for long infrared and for microwave wavelengths, can bemade by stacking parallel sheets so that the orientation directionvaries helically along the stack thickness and then densifying thestacked array so that the individual sheet thickness is about 50 nm.

Example 27

This example shows that the mechanical properties of the aerogel-likeand densified MWNT sheets are unexpectedly high, even though thesesheets are free of binder material. The density-normalized mechanicalstrength is much more accurately determined than mechanical strength,because the sheet thickness is less reliably measured than the ratio ofmaximum force to mass-per-length in the stretch direction. Stacks ofundensified sheets have an observed tensile strength of between 120 and144 MPa/(g/cm³). Mechanical property measurements for as-drawn MWNTsheet strips cut from one original sheet and stacked together so thatthey have a common nanotube orientation direction are shown in FIG. 29,parts A and B. Part A shows engineering stress versus strain, andindicates the surprisingly small variation in maximum stress for samplescontaining different numbers of sheet strips that are stacked together.The true failure stress for these samples, obtained by multiplying theengineering failure stress by the ratio of length-at-failure to initiallength, varied between 120 and 144 MPa/(g/cm³). Part B of FIG. 29 showsthe maximum force and the corresponding strain as a function of numberof stacked sheet strips for the experiment of part A. A densified stackcontaining 18 identically oriented sheets had a strength of 465MPa/(g/cm³), which decreased to 175 MPa/(g/cm³) when neighboring sheetsin the stack were orthogonally oriented to make a densified biaxialstructure. These density-normalized strengths are already comparable toor higher than the ˜160 MPa/(g/cm³) strength of the Mylar® andKapton®films used for ultra-light air vehicles and proposed for solarsails for space applications (see D. E. Edwards et al., High PerformancePolymers 16, 277 (2004)) and those for ultra-high strength steel sheet(˜125 MPa/(g/cm³)) and aluminum alloys (˜250 MPa/(g/cm³)).

Example 28

This example is also illustrates the high mechanical properties of thecarbon nanotube sheets. FIG. 30 shows an as-drawn nanotube sheetsupporting droplets of water (˜2.5 mm diameter), orange juice, and grapejuice, where the mass of the droplet is up to 50,000 times that of thecontacting nanotube sheets. The aerogel sheet regions under the aqueousdroplets are densified during water evaporation.

Example 29

This example shows a stable, planar source of polarized ultraviolet,visible and infrared incandescent light (FIGS. 31, A and B) for sensors,infrared beacons, infrared imaging, and reference signals for devicecalibration. The degree of polarization of emitted radiation for 2.5%stretched as-drawn sheets increases from 0.71 at 500 nm to 0.74 at 780nm (FIG. 32), which is substantially higher than the degree ofpolarization (0.33 for 500-900 nm) previously reported for a 600 μm longMWNT bundle with ˜80 μm emitting length. The wavelength dependence oflight intensity for both polarizations fit the functional form expectedfor black body radiation and the degree of polarization does notsignificantly depend upon sheet temperature for the observed temperaturerange between 1000 K and 1600 K. Cost and efficiency benefits resultfrom decreasing or eliminating the need for a polarizer, and the MWNTsheet provides spatially uniform emission over a broad spectral rangethat is otherwise hard to achieve. The low heat capacity of these verylow mass incandescent emitters means that they can turn on and offwithin the observed 0.1 ms or less in vacuum, and provide currentmodulated light output on a shorter time scale.

Example 30

This example shows polymer welding through heating of a transparent MWNTsheet that is sandwiched between plastic parts. The MWNT sheets stronglyabsorb microwave radiation, as evidenced by their use for this weldingof plastic parts in a microwave oven. Two 5-mm-thick Plexiglas plates(FIG. 33) were welded together using the heating of a sandwiched MWNTsheet to provide a strong, uniform, and highly transparent interface inwhich nanotube orientation and electrical conductivity are maintained.The microwave heating was in a 1.2 kilowatt microwave oven that operatesat 2.45 GHz. Input power was controlled using a water reference bylinearly ramping the water temperature to 100° C. in 3 minutes,maintaining this water temperature for one minute, and cooling thesample outside the furnace to ambient. FIG. 33 shows two 5-mm thickPlexiglas (polymethyl methacrylate) plates that were welded togetherusing microwave heating of a sandwiched MWNT sheet to provide a strong,uniform, and transparent interface in which nanotube orientation andsheet electrical conductivity is little changed. The combination of hightransparency and ultra-high thermal stability provide advantages notfound for the conducting polymers previously used for microwave-basedwelding. This microwave heating process could be used to make polymercomposites from stacks of polymer sheets that are separated by nanotubesheets, car windows that are electrically heated, and antennas in carwindows that have high transparency. Nanotube sheets can be convenientlyattached to the surface of a plastic by a related process, and inplastic by sandwiching the nanotube sheet between a low melting polymerand a high melting polymer, selected so that only the low meltingpolymer is melted as a result of the temperature increase caused bymicrowave absorption in the nanotube sheet. In these processes theplastics are chosen so that they do not provide significant microwaveabsorption in the utilized microwave frequency range.

Example 31

This example shows that by simply contacting the as-drawn MWNT sheets toordinary adhesive tape, optically transparent adhesive appliqués can bemade that could be used for electrical heating and for providingmicrowave absorption. This example further shows that these conductingpolymer appliqués can be severely bent without producing a significantchange in electrical conductivity. Conducting appliqués were made bypressing an undensified MWNT sheet (prepared as in Example 21) onto anadhesive-backed tape. Alternatively, the undensified MWNT sheet wassimultaneously attached to the adhesive backed tape and a substrateusing applied pressure. The ratio of peel strength after MWNT laminationto the peel strength without intermediate MWNT sheet was 0.7 for Al foilduct tape (Nashua® 322) on a poly(ethylene terephthalate) sheet used foroverhead transparencies and 0.9 for transparent packaging tape (3M cat.3501 L) attached to a millimeter thick Al sheet. Due to MWNT sheetporosity, the peel strength is largely maintained when an undensifiedMWNT sheet is laminated between an adhesive tape and a contacted plasticor metal surface. FIG. 34 is a photograph of an undensified MWNT sheetsused as electronically conducting and microwave absorbing appliqué. TheMWNT sheet was pressed against an adhesive tape (transparent ScotchPackaging Tape from 3M Corporation), which caused the adhesive on thetape to extrude through the pores in the MWNT sheet to provide bondingcapability to another surface (in this case a 110-μm-thick sheet ofpoly(ethylene terephthalate)). UTD is printed on a paper sheetunderneath the appliqué in order to demonstrate transparency. A movietaken of a MWNT appliqué (an undensified MWNT sheet sandwiched between atransparent packaging tape (3M cat. 3501 L) and a 110-μm sheet ofpoly(ethylene terephthalate)] shows that this appliqué can be repeatedlyfolded upon itself without causing a significant increase in electricalresistance. This ability to bend without degradation of electronicconductivity is important for flexible electronic circuits and is notfound for conventional transparent conductors like indium tin oxide. Themetal strips in FIG. 34 are electrodes used for making contact with thenanotube sheet. Going from the unfolded configuration (top picture) tothe highly folded configuration (bottom picture, in which the picturedpaper clip is used for retention of the highly folded configuration)caused little change in the inter-electrode resistance.

Example 32

This example demonstrates a method wherein transparent carbon nanotubesheets can be transformed into highly elastomerically deformableelectrodes, which can be used as electrodes for high-strain artificialmuscles and for conversion of high strain mechanical deformations toelectrical energy, and for the tunable dampening of large amplitudemechanical vibrations. The illustrative actuator material is a siliconerubber. A 1 mm thick sheet of silicone rubber (made using ECOFLEX 0040from Smooth-On, Inc.) was stretched to 105% strain, and then a single,as-drawn MWNT sheet (prepared as in Example 21) was overlaid to provideself-generated adhesive contact prior to strain relaxation. The nanotubesheet can optionally be densified by a claimed liquid-baseddensification process without undesirably effecting targeted elastomericproperties. As shown in FIG. 35, the initial sheet resistance of theobtained unloaded silicone rubber/MWNT sheet composite was 755ohms/square. However, after an initial increase in resistance by ˜6%,the resistance changed less than 3% during the subsequent four straincycles to 100% strain. Ordinary conductors cannot undergo nearly suchlarge strains without losing electrical contact with the actuatingmaterial. While conducting greases are used to maintain electricalcontact to electrostrictive actuator materials that generate 100% orhigher strains (R. Pelrine, R. Kornbluh, Q. Pei, and J. Joseph, Science287, 836 (2000)), these greases are not suitable for use as electrodesfor stacks of electrostrictive sheets that can generate large forces andhigh strains without requiring several thousand volt applied potentials.Further experimentation has shown the general applicability of thismethod of providing highly elastomeric electrodes on an elasticallystretchable substrate. For example, attachment of an undensifiednanotube sheet (prepared as in Example 21) to a 120% elongatedelastomeric Spandex® fabric (by pressing and by subsequently applyingthe liquid-based densification process of Example 23) results in ananotube electrode materials that can be elastically relaxed andre-stretched repeatedly to the initial elongation without undergoing asubstantial resistance change. More than one carbon nanotube sheetlayers can be applied on top of the Spandex layer without undesirablyaffecting the performance of the nanotube sheet layers. Suitable Spandexfibers and/or textiles are made by DuPont (and called Lycra®), DorlastanFibers LLC, INVESTA, and RadiciSpandex Corporation.

Example 33

This example demonstrates the use of the solid-state fabricated MWNTsheets as hole injecting electrodes for polymer light emitting diodes(PLEDs), which are a special type of organic light emitting diode(OLED). The free-standing, transparent MWNT sheets were fabricated usingthe solid-state process and placed on transparent glass or polymersubstrates to make either flexible or rigid PLEDs. Two active polymerswere used in the devices: PEDOT/PSS and MEH-PPV. PEDOT/PSS ispoly(3,4-ethylenedioxythiophene) (PEDOT), which is doped withpoly(styrenesulfonate) (PSS). The PEDOT/PSS was obtained as a waterdispersion containing 1-3% solids from H. C. Stark and is sold under thetrade name of Baytron® P. MEH-PPV, which was synthesized by knownmethods (C. J. Neef, J. P. Ferraris, Macromolecules 33, 2311 (2000)) ispoly(2-methoxy-5-(2′ethyl-hexyloxy)-p-phenylene vinylene). The PEDOT/PSSwas used for planarization of the as-drawn MWNT sheets during sheetdensification and as a hole transport and buffer layer, which decreasesthe barrier for hole injection from the MWNTs. The MEH-PPV served as thephoto-emissive layer. The PEDOT/PSS was first spin cast onto the MWNTsheet (at 250 rpm for a minute, and then at 760 rpm for an additionalminute to remove excess solution). After drying at 110° C. for one hour,the emissive layer of MEH-PPV was deposited in a second spin-castingprocess (at 3000 rpm for 30 seconds). After drying the MWNT sheetassembly overnight in an inert atmosphere, the device cathode was added,which consisted of a bilayer of calcium (30 nm) and aluminum (120 nm),each deposited in sequence by thermal evaporation. Luminancecharacterization of the above device showed a low turn-on voltage of 2.4volts and a maximum brightness of 500 cd/m². A similar deviceconstructed using PEDOT/PSS and MEH-PPV, but without the MWNT sheet, washighly resistive and showed no light emission. A device fabricatedwithout using a PEDOT/PSS layer (which interpenetrates the MWNT sheetand provides hole transport and planarization) showed high luminance,but required higher turn-on voltage and had a short device lifetime.FIG. 36 shows a polymer-based OLED that uses a solid-state-fabricatedMWNT sheet as the hole-injecting electrode. The transparent MWNT sheet,PEDOT/PSS, and MEH-PPV assembly covers the entire picture area, whilethe Ca/AI cathode is only on the emitting dot. The typical luminance was350 cd/m² (at 15 mA), which was further increased to 500 cd/m². Enhancedhole injection occurs due to high local electric fields on the tips andsides of nanotubes and the three-dimensional interpenetration of thenanotube sheet and the device polymers. Hole injection was limited to asingle plane for previous inorganic light emitting diodes using nanotubesheets (see K. Lee, Z. Wu, Z. Chen, F. Ren, S. J. Pearton, A. G.Rinzler, Nano Letters 4, 911 (2004)).

Example 34

This example shows that by transferring the as-drawn MWNT sheets ontonon-porous paper and densifying the as-drawn sheet, electricallyconductive printing tape comprising MWNT sheet can be made and used forprinting electrical circuits on flexible substrates. A free-standingMWNT sheet (made by the method of Example 21) was placed onto non-porouspaper (VWR 2005 catalog number 12578-121) used in the laboratory forweighing samples) and densified using the inventors' liquidinfiltration/liquid evaporation route of Example 23. Afterdensification, the aerogel nanotube sheet shrank to a thickness of ˜50nm, forming a mechanically robust electrically conductive layer. Placingthe densified sheet face down on standard writing paper and writing onthe supporting non-porous paper using a sharp object, the nanotube sheetwas transferred from the surface of the non-porous paper to the regularpaper (FIG. 37). Most important, optical microscopy on the transferrednanotube sheet regions shows that the nanotube alignment present in theprinting tape is maintained in the nanotube pattern transferred to theporous paper. Hence, the orientation of nanotubes in transferred circuitpatterns can be controlled at will be varying the relative orientationbetween the image producing and image transfer sheets. In the presentexample, the image producing sheet is the non-porous paper with theovercoated nanotube sheet and the image transfer sheet is the porouspaper upon which the nanotube sheet portions are transferred.Undensified nanotube sheet can similarly be used for printing electroniccircuit elements (simply by eliminating the densification step). Thisprocess is less attractive than the one using densified sheets, sincethe nanotubes were transferred to portions of porous paper that were notunder the writing instrument. Nevertheless, the nanotubes that weretransferred were much more firmly bound to the porous paper than thosethat were accidentally transferred, so the later could be easily brushedaway without disturbing the intentionally transferred nanotubes.

Example 35

This example shows that the nanotube sheets made by the process ofExample 21 are a type of self-assembled textile, in which nanofiberbundles branch and then recombine with other branches to form a networkhaving a degree of lateral connectivity orthogonal to the drawdirection. The SEM micrograph of FIG. 28 shows this branching and branchrecombination. Fibril deviations from draw-direction orientation arescale dependent, so they appear larger at this high magnification thanthey do at much lower magnifications. Fibril branching continuesthroughout the sheet, thereby making a laterally-extended, inherentlyinterconnected fibril network.

Example 36

This example demonstrates that free-standing solid-state fabricated MWNTsheet ribbons can be conveniently drawn and twisted to form largediameter yarns having uniform diameter. A 10.5 cm long, 3-cm-wideas-drawn nanotube sheet (made as in Example 21) was folded upon itselfalong the sheet draw direction to make a quasi-circular assembly havingabout the same length. One end was attached to the tip of a spindle andthe other end was attached to a fixed cupper wire. By introducing twist,uniform spun yarn was formed at a twist level of ˜2000 turns/meter. Thediameter of the resulting spun yarn was about 50 μm. The change inresistance from untwisted to 5000 turns/meter twist is about 12%,indicating that the interconnected fibril network provides most of theelectrical paths in the spun yarn (FIG. 47), and new contacts formed bythe twisting process are less important determinants of electricalconductivity.

Example 37

This example describes a method of draw-twist spinning of carbonnanotube yarns from a densified nanotube sheet attached to a substrate.The benefit of such process is that it enables the fabrication andstorage of nanotube sheets. Three layers of as-drawn, free-standing MWNTsheet (made as in Example 21) were placed onto a substrate (e.g., glass,plastic, or metal foil) and densified using a liquid (using a process ofExample 23). A plastic substrate, like Mylar film, was most convenientlyused. A desired width of the densified sheet was easily peeled from thesubstrate using an adhesive tape to start the draw-twist spinningprocess. By attaching one end of the peeled-off sheet strip to a motorto introduce twist at the same time that the yarn was drawn, a uniformdiameter spun yarn was obtained. This process was also extended toproduce thicker yarns, by simply increasing the number of as-drawnnanotube sheets that were initially laminated together. This number oflaminated sheets was increased from 3 to 5 and then to 8 sheets.

Example 38

This example shows the dependence of the tensile strength upon the helixangle obtained by twist as well as the major benefits of densifyingyarns by liquid treatment. If no twist is applied and the yarns are usedas drawn from the forest, the yarn mechanical strength was too low to bemeasured using Applicants' apparatus. Also, densification of drawnribbons prior to twist made it possible to obtain uniformly twisted yarnhaving uniform thickness from these ribbons even when the applied twistwas very low. Application of such low twist (corresponding to a helixangle of 5°) in the absence of pre-applied liquid-based yarndensification resulted in non-uniform twist and yarn diameter. Theobtained experimental results are shown in FIG. 48. The data points inFIG. 48 illustrated using circles correspond to yarns with diameters inthe 18-20 μm range that were spun using the method of Example 2. Inorder to obtain a mechanical strength high enough for measurement, thedata for zero twist (indicate by the square) was obtained for a yarnthat was densified using liquid treatment. The untwisted yarn wasobtained from a ˜5-mm wide ribbon that was drawn from the forest ofExample 1, and then densified using ethanol (like done for nanotubesheets in Example 23). While this rather wide ribbon was employed in thepresent example, the described strengthening effect also results formuch narrower yarns. The effect of this liquid treatment (involvingliquid imbibing, and filament densification during liquid evaporation)was to dramatically increase strength, as well as to increase tenacity.The resulting yarn exhibits a non-circular cross section. Since the yarnwith 5° twist could not reach uniform twist along its length, thissample was also densified using the ethanol treatment after twist wasapplied. As shown in FIG. 48, a peak tensile strength of ˜340 MPa isachievable with a helix angle of around 20°. In the absence of liquidpre-treatment (like described in Example 23 for nanotube sheets), thedata point in FIG. 48 for zero twist would be near zero on the tensilestress scale. Also, in the absence of solvent densification, the datapoint for 5° twist would correspond to a greatly reduced strength. Toomuch twist also decreases strength, as shown in FIG. 48. Adding 70degrees of twist to an untwisted yarn reduces its tensile strength byone-half. Although highly twisted yarns have decreased failure stresses,they have higher failure strain than for more moderately twisted yarns,as is indicated in FIG. 49 (which the tensile strain measurements forthe same sample trials as in FIG. 48). Very importantly, the data inFIG. 48 (square data points) shows that useful strengths can be obtainedby liquid treatment even when twist has never been applied.

Example 39

This example shows that yarn's tensile strength decreases as the yarndiameter increases. The yarn spinning method is like that of Example 2.The yarn diameter is controlled by the width of the ribbon from which itis spun and the data points in FIG. 50 correspond to ribbon widths inthe 3-27 mm range. Since the yarn's tensile strength is dependent oftwist angle, twist angle was maintained constant while varying the yarndiameters. As shown in FIG. 50, higher twist (˜50°) yarns (triangle datapoints) are less susceptible to this decrease in strength withincreasing yarn diameter than are those with lower twist (˜15°) yarns(circle data points). The failure strain exhibits a much weaker ornegligible dependence on the yarn's diameter. The yarns consistentlybreak at around 10% strain (FIG. 51) independent of the yarn diameter.

Example 40

This example shows that twist dramatically increases yarn tensilestrength, even when this twist is subsequently eliminated by an equaltwist in an opposite direction. Both yarns shown in FIG. 52 wereprepared from the same nanotube forest (using the method of Example 2)and the same width of ribbon. Hence, the number of fibrils passingthrough a yarn cross-section was maintained. The twist for Yarn A is26000 turns/m clockwise, resulting in a twist angle of 280. For Yarn B,a 26000 turns/m clockwise twist was introduced first forming alock-stabilized yarn and then the same twist was introducedanticlockwise to release all the twist. Note that the increase in yarndiameter (compare A and B SEM micrographs of FIG. 52) as a result oftwist de-insertion is relatively small. The tensile forces at break are24 mN for Yarn A and 14 mN for Yarn B. The resulting tensile strengthsare 339 MPa for Yarn A and 113 MPa for Yarn B. Since the tensilestrength of a never-twisted yarn is too low to even be measured inApplicants' apparatus, these measurement results indicate that the neteffect of twisting and untwisting is to dramatically increase tensilestrength, and that retention of twist can further increase tensilestrength. Since strong untwisted yarns are highly desirable for use informing nanotube/polymer composite yarns have both high strength andhigh toughness, this surprising discovery that false twist (twistinsertion followed by twist de-insertion) can dramatically increase yarnstrength is quite important. This discovery provides the motivation forthe false-twist spinning apparatus described in FIGS. 44-46.

Example 41

This example illustrates that carbon nanotube (CNT) sheets can be drawnfrom a forest, attached to a substrate film (such as a plastic, metalfoil, or Teflon film), densified, and wound onto a mandrel.Demonstration of the feasibility of this process for adhesive-free,adhesive-coated, and elastomeric substrates is provided in Examples 23,31, and 32, respectively. FIG. 53 and FIG. 54 show schematicillustrations of such processes. Element 5302 in FIG. 53 is a nanotubeforest prepared as described in Example 1. Element 5301 is a growthsubstrate, element 5303 is a nanotube sheet drawn from the forest,element 5304 is the substrate film, and element 5305 is nanotube sheetattached to the substrate film. The attached nanotube sheet is densifiedusing a liquid (element 5306), dried by a heater (element 5307), andthen wound onto a mandrel. Rollers (two) are represented here by opencircles and mandrels (three) are represented by filled circles. Byrepeating the process, multilayer of nanotube sheets can be applied tothe substrate film. A variation of the process is illustrated in FIG.54. Instead of using liquid, liquid vapor (element 5406) is used todensify the collected sheet and the densified sheet (element 5407) iswound onto a mandrel. The elements are nanotube forest substrate (5401),nanotube forest (5402), CNT sheet (5403), substrate film (5404), CNTsheet attached to substrate film (5405), a heating system for deliveryof vapor (5406), densified CNT sheet on substrate film (5407), substratefilm delivery mandrel (5408), roller for consolidation of nanotube sheetand substrate film (5409), and collection mandrel (5410). Each of therollers in FIGS. 53 and 54 can optionally be replaced by pairs ofrollers, one on each side of laminated nanotube sheet and substratefilm. Importantly, the densified nanotube sheet produced by theapparatus of FIGS. 53 and 54 can be later unwound from the mandrel andseparated from the substrate film for the twist-based spinning of yarn(see Example 37), for forming free-standing densified sheets or formechanical transfer of selected portions of nanotube sheets to othersubstrates (see Example 34). Also, the substrate can be an elastomericfilm (or textile) that is stretched prior to attachment of the nanotubesheet (see example 32) or an adhesive coated substrate sheet (seeExample 31). The stretching can be accomplished by controlling therelative rotation rates of substrate delivery and substratefilm/nanotube sheet take-up mandrels and rollers (or roller pairs)between these mandrels.

Example 42

This example illustrates a process by which a nanotube sheet can bedrawn from a forest, attached to an adhesive-coated substrate film,sealed with a second film (such as a plastic, metal foil, or Teflonfilm), and wound onto a mandrel. FIG. 55 shows a schematic illustrationof the process. Element 5502 in FIG. 55 is a nanotube forest prepared asdescribed in Example 1. Element 5501 is a growth substrate, element 5503is a nanotube sheet drawn from the forest, element 5504 is the adhesivecoated film 1, and element 5305 is nanotube sheet attached on theadhesive film 1. The attached nanotube sheet is sealed with a film 2(element 5506), and the sandwiched nanotube sheet (element 5507) iswound onto a mandrel. Film 2 can be later separated from the nanotubesheet/film assembly. Because of the porous nature of nanotube sheet,tape adhesiveness remains and the adhesive sheet with attached nanotubesheet can be conveniently applied to any desired surfaces. In FIG. 55,rollers are represented by open circles and mandrels are represented byfilled circles.

Example 43

This example illustrates a method for spinning a single carbon nanotubeyarn from the sides of the two nanotube forests. Two nanotube forestswere placed in close proximity, so that the tops of these forestsprovided either inter-forest contact or the closest approach between thenanotube forests. Narrow nanotube ribbons were simultaneously drawn fromthese two stacked forests, attached to the tip of a spindle, andsimultaneously twisted and drawn to provide a single unplied nanotubeyarn.

Example 44

This example shows a method of initiating draw of a nanotube sheet.First, a straight line is scratched on the back side of silicon wafersubstrate that is optionally used for nanotube growth, the wafer isbroken into two forest sections, and then these two forest sections areseparated preferably in a direction orthogonal to the original scratchdirection. Since nanotube bundles are interconnected in the forest,nanotube sheet forms between the two sidewalls of the forests. In thisconfiguration, the allowable sheet production rate can be doubledbecause nanotubes are fed from both sides. The scratch line can be madebefore or after forest growth.

Example 45

This example demonstrates a method wherein stacks of carbon nanotubesheets can be deposited on a contoured surface and densified on thissurface, so that the shape of the contoured surface is retained in theshape of the nanotube sheet array. This application demonstrationenables, for example, the deposition of carbon nanotube sheets as alayer in a contoured composite (such as an aircraft panel), as acontoured heating element for deicing on an air vehicle, or a contouredsupercapacitor that provides both an energy storage and structuralcomponent for a contoured car panel. The contoured surface used for thisdemonstration was an oval shaped plastic bottle. The long and short axisof the oval cross-sectional area went from 4.1 cm by 2.7 cm at the baseto 3.6 cm by 1.8 cm at a height of 4.3 cm from the bottle base. Thissection of this lower part of the bottle was used for the contouringprocess. An absorbent material (a single ply of a two-ply cellulosefacial tissue) was wrapped around the bottle. After wetting the tissuewith isopropyl alcohol, an undensified 4 cm wide nanotube sheet(prepared as in Example 21) was wrapped around the sides of the bottlebase (so that the nanofiber orientation direction is circumferential)and the isopropyl alcohol was allowed to dry, thereby densifying thenanotube sheet in the shape of the bottle. This process was repeatedtwenty to thirty times to produce a densified stack of nanotube sheetlayers having the shape of the lower region of the bottle. Upon removalof the nanotube sheet and attached tissue paper sheet from the bottlemandrel, it was observed that the nanotube sheet stack maintained theshape of the bottle. Moreover, this contoured shape recovered afterforcing the sheet stack to be planar by applying a load, and thenremoving this load.

Example 46

This example describes methods for initiating the drawing of a nanotubesheet, ribbon, ribbon array, yarn, or yarn array from a nanotube forestusing an adhesive, an array of pins, or a combination of an adhesive andan array of pins. Interestingly, the inventors find that contact of anadhesive tape to either the top or sidewall (edge) of the nanotubeforest is useful for providing the mechanical contact that enables thestart of sheet draw. Using nanotube forests prepared as in Example 1,various adhesive types worked well for initiating sheet draw, includingthe adhesive attached to a 3M Post-it Note, Scotch Transparent Tape (600from 3M), Scotch Packaging Tape (3M 3850 Series), and Al foil duct tape(Nashua 322). Contact of a straight adhesive strip (so that the adhesivestrip is orthogonal to the draw direction) worked especially efficientlyto start the draw of a high structural perfection sheet. The reason thatthis top contact method is especially advantageous is that nanotubeforests typically have non-straight sidewalls, and the use of a straightadhesive strip (or a straight array of suitably spaced pins) providesstraight contact for the forest draw. An array of closely spaced pinswas also usefully employed to start sheet draw. In one experiment, thepin array consisted of a single line of pins. The mechanical contactneeded for spinning was in this case initiated by partial insertion ofthe linear pin array into the nanotube forest. The pin diameter was 100micron, the pin tip was less than one micron, and the spacing betweenthe edges of adjacent pins was less than a millimeter. Satisfactorysheet draw was achieved using pin penetration of between ⅓ and ¾ of theheight of the forest (in the range between 200 and 300 microns). Drawprocesses of multiple ribbons or yarns can be similarly initiated usinga linear array of adhesive patches or a linear array of pins that areseparated into segments. The separation distance between adhesivepatches along the length of the linear array determines the ribbon widthor the width of the sheet strip used to make the yarn (by, for instance,twist-based spinning, false-twist spinning, liquid-densification-basedspinning, or any combinations thereof). Mechanical separation of thesheet strip patches or the pin patches in the linear array during thestart of draw is usefully employed in order to avoid interference duringprocessing of adjacent ribbons or yarns, such as during the introductionof twist. Use of adhesive patches (or pin patches) that have differentlengths along the strip direction can be usefully employed—such as todraw-twist adjacent strips to produce different diameter yarns (whichcan be optionally combined to provide a plied yarn in which differentsingles yarns in the plied yarn have different diameters). Differentdegrees of twist or directions of twist can be conveniently and usefullyapplied to different singles yarns that are drawn using the segmentedadhesive or pin strip, and these different singles yarns can then beoptionally plied together in a yarn containing a freely-selected numberof plies. Importantly, using the above methods for introducing differentdiameter singles yarns into a plied yarn can be employed to produce aplied yarn having enhanced density, since smaller diameter single yarnscan help fill-in void spaces between larger diameter singles yarns.

Example 47

This example shows that the spun nanotube yarns of invention embodimentscan be easily inserted into conventional textiles, to thereby provideelectrical interconnects, sensors, and other electronic elements inthese textiles. The optical micrograph of FIG. 42 shows a two-ply MWNTyarn (comprised of 12 μm diameter singles yarns) that has been insertedin a conventional fabric comprising 40 Lm diameter melt-spun filaments.The insertion method was accomplished by tying the nanotube yarn (madeby the process of Example 4) onto the end of a filament in the originalfabric, and pulling this filament out of the fabric as the nanotube yarnis pulled into the fabric.

Example 48

This example demonstrates a novel continuous spinning apparatus forspinning fine and ultra-fine nanofiber yarns, which introduces twist asit winds the spun yarn onto a bobbin. The apparatus is shownschematically in FIG. 38 and described in Section 5 on “Elaboration onTwist Insertion and Filament Storage Methods during Spinning”. Thespinning apparatus in this example comprises a spindle, a donut-shapedwinding disk with an associated winding yarn guide, an electromagnet,and a donut-shaped metal magnetic disk, which contacts the ferromagneticspindle base, which is typically made of steel. The diameter ofdonut-shaped winding disk is 20 mm with a thickness of 3 mm. The robinsused are typically 5 mm in diameter and 30 mm long. The spindle isdriven by a variable-speed DC motor, which is controlled through acomputer interface and has a maximum speed of 15,000 rpm. Anelectromagnet is used to introduce a variable braking force onto thewinding disk, which reduces its angular speed relative to the spindle.The rotation of the drafted nanofiber assembly about the axis of thespindle introduces twist, thereby forming the yarn, while the slowerrotation of the winding disk winds the spun yarn onto the spindle.Advantageously, both twist level and spinning speed can be independentlycontrolled by an electronic interface to independently regulate motorspeed and applied magnetic field. This system imposes minimal tension tothe spun yarns and can handle spinning of yarns with either high or lowbreaking force. This same apparatus can also be utilized to ply multiplesingle-strand yarns together to continuously make multi-strand yarns. Insuch cases nanotube forest is replaced by reels of unplied yarn.

Example 49

The inventors find that too low a density of nanotubes in a forestrenders a forest difficult to spin as a yarn or draw as a ribbon orsheet. This is illustrated in FIG. 56 where SEM micrographs of thegrowth substrates are compared for spinable and practically non-spinablenanotube forests (after removal of the nanotubes), wherein the smalldiameter pits on the growth substrate correspond to the growth site ofMWNTs. The nanotube diameters (about 10 nm) are roughly the same forboth of these spinable and practically non-spinable forests. However,the inventors observed (by counting the pit densities on the growthsubstrate) nanotube forest base area densities of 90 billion to 200billion nanotubes/cm² for nanotube forests that are highly spinable, ascompared with 9 billion to 12 billion nanotubes/cm² for low densitynanotube forests that were difficult or impossible to spin or draw.Also, the inventors observed that the percentage of the forest base areathat was occupied by nanotubes was much higher (7% to 15%) for highlyspinable forests, as compared with 1.1% to 2.5% for nanotube foreststhat were difficult or impossible to spin.

Example 50

This example demonstrates that nanotube sheets can be deposited on asubstrate, densified by using the liquid infiltration method, and thenpeeled from the substrate to provide a free-standing, densified sheetarray. The importance of this demonstration is that it enables thestorage of densified nanotube sheets on a mandrel, and subsequentretrieval of these densified sheets from the sheet substrate (typicallya plastic film carrier) for applications. Either three, five, or eightlayers of as-drawn, free-standing MWNT sheet (made as in Example 21)were placed onto a substrate (e.g., glass, plastic, or metal foil) anddensified using a liquid (using a process of Example 23). A plasticsubstrate, like Mylar film, was most conveniently used. Any desiredwidth (or the entire width) of the densified sheet was easily peeledfrom the substrate using an adhesive tape to start the sheet removalprocess.

Example 51

The example shows that very thin densified carbon nanotube sheet stacks(down to less than 150 nm in thickness) can be rolled onto a mandrel forstorage and possible shipment, and then subsequently unrolled forapplication without supporting the nanotube sheets with a carrier sheet(like in the Mylar film in Example 50) during this operation. Thisdemonstration was provided by pressing two free-standing densifiednanotube sheet stacks together (after they were peeled from the Mylarfilm substrate in the process of Example 50) and observing that the twonanotube sheet stacks had no significant tendency to stick together.

Example 52

This example shows that liquid densified nanotube sheet stacks can beformed on cellulose tissue paper, that the nanotube sheets or ribbonscan be easily peeled from this cellulose substrate, and that theseribbons can be twist spun to make strong nanotube yarn. The investigatedsheet stack/tissue laminate was made as in Example 45. Despite the factthat the nanotube sheet stacks were contoured as a result of beingformed on a mandrel having oval cross-section, either the entirenanotube sheet stack width (4 cm) or narrow ribbons could be uniformlypulled from the cellulose tissue substrate. Ribbons pulled from thetissue substrate (3 mm and 5 mm in width) were twist spun to make strongnanotube yarn.

Example 53

This example demonstrates a twist-based method for making a fibercomposite of two different fibrous materials, one comprisingelectronically conducting carbon nanotubes and the other comprisingelectronically insulating cellulose microfibers. Also, this exampledemonstrates a method that provides either the insulating microfibers orthe conducting carbon nanofibers on the outer surface of the twistedyarn. In addition, this demonstration shows how a carbon nanotube yarncan be covered with an insulating layer. Also, by replacing thecellulose sheet with a similar sheet comprising fusible polymermicrofibers (such as polypropylene or polyethylene-based non-wovenpaper), the methods of this example can be used to make polymer/nanotubecomposite yarns that are either twisted or false twisted prior to fusionof the polymer onto the nanofibers in the yarn by thermal or microwaveheating. This demonstration uses the tissue paper/nanotube stackcomposite that has been contoured using the method of Example 45. Threemillimeter width ribbons were cut parallel to the nanotube orientationdirection from the composite stack, and twisted to provide a moderatestrength yarn. Apparently, because of the contouring on the oval mandrel(with the nanotube fiber direction in the circumferential direction),the inventors found that (depending upon the direction of twist) eitherthe insulating cellulose microfibers or the electrically conductingcarbon nanofibers would appear on the surface of the twisted yarn.

Example 54

This example describes the application of highly anisotropic carbonnanotube sheets for making a bolometer in which the nanotube assembly isused as a heat delivery materials rather that as a sensing material. Anextremely high thermal diffusivity (D of above 0.1 m²/s) and a highthermal conductivity of solid-state drawn MWNT nanofiber sheets (K=50W/mK) of the present invention (see preparation in 21) allows for rapidhighly anisotropic transfer of temperature fluctuations to column and/orrow electrodes of a matrix addressable sensor with minimal energydissipation. The electrical signal(s) are established through heating atthe nanotube sheet-electrode interface. A process of forming a thermalimaging display is shown in FIGS. 59 A-C. FIG. 59 A shows afree-standing MWNT sheet 5902 suspended between two holders 5901 and5903. Substrate 5904 of FIG. 59 B (which serves to frame and support twoorthogonally oriented nanotube sheets) is formed of an insulatingdielectric material, like polycarbonate or a fiber glass sheet. Rowmetallic electrode pads 5906 are formed on top of substrate 5904. Thecolumn metallic electrode pads 5905 are formed on the other side ofsubstrate 5904. A temperature sensing layer 5908 is formed on electrodepads of a material responsive to temperature fluctuation. In someembodiments, the temperature-sensitive layer is formed of a vanadiumdioxide (or other suitable semiconductor material) that exhibits a hightemperature coefficient of resistivity at room temperature and changesthe series resistance of the entire circuit comprising: metallicelectrode/semiconductor layer/nanotube assembly/semiconductorlayer/metallic electrode. In other embodiments, the temperaturesensitive layer 5908 is formed of a thermocouple material (e.g., iron)on one side of the electrodes and constantan on another. The carbonnanotube assembly delivers the heat to both ends. The thermocouplelayers convert the temperature fluctuation into a thermoelectricpotential. Once the electrodes are formed, highly-aligned MWNT nanofibersheets are attached to both sides of the substrate, positioning thenanotube alignment of each respective sheet orthogonal to the other. Thehigh anisotropy of thermal and electrical conductivities of suspendedMWNT sheets minimizes lateral cross-talk between nanofiber orientationand orthogonal directions and permits high resolution thermal imaging ofheat-radiating objects.

Example 55

This example serves to illustrate how extremely low 1/f (f-frequency)noise and a low thermal coefficient of resistivity (see Example 25)enable application of MWNT carbon nanotube sheets as precisionresistors, in accordance with some embodiments of the present invention.A schematic of the device is shown in FIG. 60. The free-standing MWNTsheet 5902 (optionally prepared using the process of Example 21) issuperimposed on the substrate 6004, which comprises metallic electrodes6001 and 6002, prior to the deposition of these electrodes by optionallyscreen printing. Substrate 6004 is a supporting dielectric materialhaving a low thermal expansion coefficient, such as alumina. After theattachment of sheets to the substrate, the entire device is liquiddensified using the process of Example 23. After drying, the filmexhibits strong adhesion to the substrate surface. To avoidenvironmental influences on the resistance, the nanofiber sheet resistoris preferably packaged using technology similar to that used for organiclight emitting organic diodes. The as-fabricated resistor exhibits anextremely low temperature coefficient of resistivity (α=7.5×10⁻⁴ K⁻¹)and low 1/f noises.

Example 56

This example illustrates the fabrication of a screen for electromagnetic(EM) shielding, in accordance with some embodiments of the presentinvention. A schematic illustration of such an EM shielding device isshown in FIG. 61. This EM shielding screen can be optionally fabricatedas a free-standing transparent sheet using a solid-state nanofiber sheetdraw process (Example 21), attached to a surface (such as a displayscreen surface using a sheet deposition process like in Example 18, andan optional sheet densification process like that shown in Example 23).Nanotube orientation can optionally be aligned along the patterningdirection of a front transparent electrode of the display screen inorder to obtain efficient EM shielding. Alternatively, the EM shield cancomprise nanotube sheet stack in which orientation directions of sheetstacks are not in the same direction (for the purpose of eliminatingpolarization effects for the screen.

Example 57

This example demonstrates the use of a highly aligned and porous MWNTnanofiber sheet as a host material for a gas sensor, in accordance withsome embodiments of the present invention. MWNTs are not by themselveshighly sensitive to adsorbed gases. However, a suspended sheet of MWNTsis mechanically strong, has high porosity, and has very anisotropicconductivity along and perpendicular to the nanotube alignmentdirection. The anisotropy factor is usually above 20, but it can beeasily increased by mechanical treatment to above 100. To sensitize theMWNT sheet, SWNTs from suspension are deposited onto the MWNT sheet(such as by SWNT deposition from a liquid dispersion) or by growth ofthe SWNTs on the MWNT sheets (such as by known CVD processes). In oneembodiment (illustrated in FIG. 62 A), the MWNT sheet is deposited on adielectric substrate (such as a glass or ceramic) on which there arepreviously deposited electrodes 6201, 6202, 6203, and 6204. In anotherembodiment, an opening in the substrate allows gas to flow through theporous MWNT sheet comprising the deposited SWNTs. As a result, theconductivity across the MWNT sheet is determined by the conductivity ofthe deposited SWNTs 6206 which are very sensitive to the gas environmentdue to their intrinsic properties. FIG. 62 B demonstrates the highsensitivity of SWNT sheets (filtration produced HiPco nanotube paper) tothe vapor of benzene and alcohol. After each exposure to the gas andmeasurement cycle, the sensing surface of the device is recovered byheating the MWNT sheet to optionally about 300° C. by applying currentalong the host MWNTs, i.e., to electrodes 6201 and 6202 of FIG. 62 A.

Example 58

This Example illustrates an application whereby nanotube sheets andyarns are used as antenna, in accordance with some embodiments of thepresent invention. Such antenna comprising the nanotube sheets ofExample 21 can be transparent, which is also the case for antennas basedon sparsely weaving the nanotube yarns of invention embodiments intotransparent textiles. The optically transparent antennas can belaminated on structures that need to be viewed optically, such asradio-frequency identification (RFID) tags overlaid on bar codes,pictures, photo-frames, displays, etc. In some such above-describedembodiments, the antenna have a structure like that shown in FIG. 63,where such an antenna is composed of a microstrip line type feeder, madeof a copper line or any other conductive strip, which can be alsotransparent, like another nanofiber sheet or ITO film. In someembodiments, a thin insulating layer of polymer, 5-10 microns thick, iscoated on the top of a microstrip line feeder and the antenna reflectorplane, which is made of a single nanofiber sheet, or several sheets withyarns for better conduction is laminated on top of it. The resonancewavelength of the antenna can be tuned by the length of the nanofiberelement L, which has an average thickness derived from the nanofiberforest array, from which the nanofiber sheet or yarn is optionally spun.

Example 59

This example serves to illustrate a heat exchanger based on MWNTnanofiber sheets. Drawn from a MWNT forest, such suspended sheetsexhibits extremely high thermal diffusivity and high thermalconductivity. Densification of the sheet on a substrate dramaticallydecreases the heat transport by two orders of magnitude because ofimproved heat dissipation to the substrate. Meanwhile the thermaltransport along the nondensified sheet remains very high. In thisexample, the use of a MWNT sheet as a heat exchanger for a computermother board is described. The high electronics density laptop computercontains several highly-integrated processors distributed on the motherboard. It is impossible to provide each processor with its own coolingfan, such as is often used for the main processor in desktop computers.Laptop computers usually use a copper heat exchanger to direct all heatto a metallic plate cooled with a fan cooler built into some corner ofthe computer body. The thermal diffusivity of MWNT sheets is two ordersof magnitude higher than that of bulk copper. As shown in FIG. 64, toimprove the heat exchange, Applicants propose that the copper plate 6403be covered with MWNT sheet 6401 to enforce the delivery of thermalenergy to the cooler 6404 built into some corner of the computer body.The surface area above the contact with processor (or other heatsources), 6402 and fan cooler are preferably densified to improve theheat exchange between the copper plate and the MWNT sheet. The heatdelivery bus can optionally be un-densified.

Example 60

This example shows that Field emission properties of the twisted MWNTyarns were studied in the single end geometry (FIG. 66 B). MWNT yarns of10 μm diameter were obtained using the draw-twist spinning process ofExample 2, and automated versions of this process (such as shown inExample 48). One end of a 6 mm long yarn was affixed to a flat nickelplate using conductive tape that is used for scamming electronmicroscopy. Thus, a 5 mm MWNT yarn that is free-standing at an arbitraryangle was obtained. The resulting MWNT cathode so produced wasintroduced into a vacuum chamber along with a tungsten anode plate andthe chamber was pumped down to 10⁻⁷ Torr. The distance between thenickel plate and the tungsten anode plate was 10 mm. A short (1 ms) highvoltage (2000 v) conditioning pulse was applied to raise the MWNT yarnup to roughly vertical orientation. Subsequent to that, I-V measurementswere performed in the DC regime. FIG. 67 shows an I-V curve for thefield emission of the yarns in the single end geometry. The three stagesof field emission are easily seen. At low voltages, no current isobserved except noise from the measurement system. From 700 V to 1200 V,the field emission seems to obey Fowler-Nordheim (FN) law (R. Gomer,Field Emission and Field Ionization, Harvard University Press,Cambridge, Mass., 1961, Chaps. 1-2). Above 1200 V, the FE I-V curvedeviates considerably from FN law, which may be attributed to somesaturation of current due to an adsorbate-enhanced field emissionmechanism.

Example 61

This example serves to illustrate light emission from a verticalgeometry for a single yarn end-tip cathode, in accordance with someembodiments of the present invention. In a single yarn end-tip geometryincandesced light emission was observed from the end of the yarn whenthe applied voltage exceeded 1460 V (see FIG. 68, inset). Such emissionis attributed to Joule heating. FIG. 68 shows the spectra of theincandesced light, wherein the temperature is about 2200 K based on thecolor of the emitted light.

Example 62

This example serves to illustrate electron emission from a planargeometry configuration for a nanofiber yarn cathode, in accordance withsome embodiments of the present invention. From a MWNT yarn of 10 μmdiameter and meter length, a 12 mm long segment was extracted andconnected to a flat nickel plate using electrically conducting SEM tapeon both ends. So taped, about 10 mm of said yarn remained uncovered. Theprepared sample was introduced into a vacuum chamber along with atungsten anode plate, and the chamber was pumped down to 10⁻⁷ Torr. Thegeometry of the experiment is depicted in FIG. 66 A. The field emissionproperties of the lateral side of the 10 mm long segment of exposed yarnwere examined in the DC regime with a Keithley 237 unit. FIG. 69 showsI-V plots for lateral electron field emission from the MWNT yarn.Typical I-V (current (I) versus voltage (V)) plots in this geometry showhysteresis behavior. Initial (first rise) I-V plots go considerablysteeper and have a large threshold field value (0.9 V/μm) than thoseobserved in the following set of measurements (0.7 V/μm). Thishysteresis behavior may be a result of electrostatic forces which touslethe MWNT on the yarn, thereby making it fuzzier. After this, theFowler-Nordheim plots became repeatable and may likely be interpretedwithin the framework of the Fowler-Nordheim theory of cold cathodeelectron emission. Once a rather high field was applied, on the order ofabout 5-8 kV for 10-15 hours, the electrostatic hairy forest evolvedfurther protrusions of nanotubes. This resulted in the formation ofhairy parts on the MWNT yarn surface (see FIGS. 74 A and B). The samesaid lateral geometry was also used with a phosphor screen anode insteadof a tungsten plate. Typical images of field emission from said yarns inthis lateral geometry taken with a phosphor screen anode are depicted atFIGS. 70 and 71. To avoid rapid phosphor burning in the DC regime, thenegative pulsed voltage was applied with a HV Pulse M25k-50-N unit tothe cathode. The repetition rate of the pulses was 1 kHz and the dutycycle was 1%. It is clearly seen that the field emission properties ofthe MWNT yarn are very uniform. Moreover, at certain applied voltages,the field emission sites cannot be distinguished. This property ofuniform lateral emission from the MWNT yarn may be widely used indifferent types of flat panel displays and indicators, such as lettersand/or alpha numeric code numbers (shown in FIG. 71).

Example 63

This example describes the application of transparent MWNT nanofibersheets as a cold cathode for electron emission. A nanotube fiber sheethas been prepared by the above-described of Example 21, and was placedon a square piece of glass (25 mm×25 mm, 1 mm thickness) and thendensified two times in methanol using the densification method ofExample 23. After that, all of the borders of the nanotube sheet werecovered with SEM electrically conducting tape, such that a 10 mm borderstrip of said sheet was fully covered. An uncovered square surface of 5mm×5 mm was left. FIG. 79 is a schematic illustration of a flat coldcathode prepared this way. The cathode was introduced into a vacuumchamber along with a tungsten anode plate and pumped down to 10⁻⁷ Torr.Field emission properties of the uncovered square of MWNT transparentsheet cathode were studied with a Keithly 237 unit. The distance betweenthe cathode and tungsten anode plate was 250 μm. I-V plot of the fieldemission of said MWNT transparent sheet shows that the threshold field(when a field emission current reached 100 nA) was 0.8 V/μm.

Example 64

This example demonstrates the application of transparent nanotube sheetsfor a conventional display technology wherein the phosphor screen isbetween the electron emitting elements and the viewer (like for FIG.82). A cathode prepared as in the previous example was introduced into avacuum chamber along with a phosphor screen anode and the chamber waspumped down to 10⁻⁷ Torr. The phosphor screen comprised ITO coated glassfurther coated with a “green” TV phosphor. The cathodoluminescent imageswere observed through the glass window of the chamber. In this example,the phosphor screen was placed between the cathode and the glass windowof the chamber (like for the schematic illustration of FIG. 82). Thedistance between the cathode and the phosphor screen anode wasapproximately 200 μm. Typical images of field emission from said coldcathode taken with the phosphor screen anode show a quite nonuniformintensity distribution, probably due to variation of density ofend-tips. To avoid rapid phosphor burning in the DC regime, thenegatively-pulsed voltage was applied to the cathode by using a HV PulseM25k-50-N power supply. The repetition rate of the pulses was 1 kHz andthe duty cycle was 1%. Typical negative pulse amplitudes that wereapplied were between 300 V and 3 kV.

Example 65

This example demonstrates a novel type of display technology orinvention embodiments in which a transparent nanofiber sheet cathodeelectron emitter separates the phosphorescent light emitting layer fromthe viewer. A cathode as prepared in the previous example was introducedinto a vacuum chamber along with the above-described phosphor screenanode, and the chamber was pumped down to 10⁻⁷ Torr. A transparentnanotube sheet cold cathode was placed between the phosphor screen andthe glass window of the chamber (as shown at FIG. 83). Typical images offield emission from said cold cathode taken with the phosphor screenanode are shown in FIG. 71. The details of voltage pulsing and theutilized power source were the same as for Example 64.

Example 66

This Example serves to illustrate a carbon nanotube-based polymeric LEDdevice: PLED on a flexible plastic, in accordance with some embodimentsof the present invention. Such a device can comprise the followingstructure: carbon nanotube (CNT)sheet/PEDOT:PSS/MEH-PPV/calcium/aluminum, similar to device described inExample 32. To make the above-described device, a free-standing carbonnanotube (CNT) nanofiber sheet (shown as 8505 at FIG. 85) is placed on asubstrate (8507) of flexible plastic (polyethylene terephthalate (PET)or poly(ethylene-2,6-naphthalate (PEN)). The undensified sheet is weaklybound to the substrate and can be rubbed off very easily. Care must betaken during subsequent processing so that this does not occur. Thesubstrate with the CNT sheet on top is then densified. The densificationprocess is performed by placing the substrate into a beaker of methanolor ethanol. The substrate is held vertically and dipped into the solventalong the direction of the sheet orientation. The substrate is thenplaced onto a cloth and allowed to dry. This procedure of dipping anddrying is performed two more times. The CNT sheet has now been densifiedand has a thickness of approximately 200 nm. Several layers of thehole-injecting polymer PEDOT:PSS (shown as 8504) are deposited after thesolvent has been dried from the densified CNT sheet. PEDOT:PSS (BayerCo.) is filtered prior to depositing onto the CNT substrate. A firstlayer is deposited at a spin rate of 6100 rpm, with an initialacceleration of 21,500 rpm/second for 20 seconds. A plastic syringe isfilled with a solution of the PEDOT:PSS in water. The spinner is startedand the solution is immediately dropped onto the substrate whileacceleration is still occurring. This produces a thin film of PEDOT:PSS.The substrate is then baked at a temperature of 120° C. for 30 minutes.A second layer of PEDOT:PSS is deposited using the a spin process with aslower acceleration of 160 rpm/second, but the same target rate of 6100rpm and time of 20 seconds. The solution is dropped onto the substrateprior to the start of spinning, however, unlike the first film for whichspinning was started before dropping the solution on. This film is thenbaked under the previous conditions of 120° C. for 30 minutes. Third andfourth layers are deposited using the same spinning conditions as usedwith the second layer (6100 rpm, 160 rpm/second acceleration, 20seconds) and baked after each deposition for 30 minutes at 120° C. Theemissive layer of the device (8503 layer) is produced from a solution of0.2 wt % MEH-PPV in chloroform. The MEH-PPV solution is dropped (i.e.,deposited) onto the substrate containing the CNT sheet and PEDOT:PSSfilms. Spinning is done at a rate of 3000 rpm, with an initialacceleration of 3400 rpm/s for 30 seconds. Spinning is started and theMEH-PPV/chloroform solution is immediately dropped onto the acceleratingsubstrate from a plastic syringe. A single film of MEH-PPV is producedon the device. The MEH-PPV film is dried overnight in an argon-filledglove box. The device fabrication is finalized by depositing cathodesonto the CNT/PEDOT:PSS/MEH-PPV structure of FIG. 85. The deposition ofthe cathodes is performed in a high vacuum chamber equipped with thermaldeposition equipment. Calcium (8502 in FIG. 85) and aluminum (8501) aredeposited in a single pump-down cycle from separate sources. The basepressure in the chamber prior to the start of deposition is <2×10⁻⁶Torr. A shadow mask is used to place the cathodes in the desiredlocations on the substrate. The calcium is deposited first at an initialrate of 0.5 Å/s second. After 100 Å of Ca has been accumulated, the rateis ramped at 2 Å/s over a 60 second period. This rate is held until 300Å of calcium have been deposited. Aluminum (Al) is then deposited at aconstant rate of 5 Å/s until 1200 Å have been deposited. Testing showsresults similar to those described in Example 32

Example 67

This Example serves to illustrate a carbon nanotube-based small-moleculeOLED on display glass or flexible plastic, in accordance with someembodiments of the present invention. This device is similar to thepreviously-described polymeric devices, however, it incorporatesthermally-evaporated molecular films in place of polymeric films as theactive layers. The device structure is as follows: carbon nanotube (CNT)sheet/PEDOT:PSS/α-NPD/Alq₃/Aluminum shown in FIG. 86. To fabricate theabove-described device, a free-standing carbon nanotube (CNT) sheet(8606) is placed on a substrate of Corning 1737 display glass (8607) orflexible plastic (PET or PEN 8608)). The sheet is weakly bound to thesubstrate and can be rubbed off very easily. Care must be taken duringsubsequent processing so that this does not occur. The substrate withthe CNT sheet on top is then densified. The densification process isperformed by placing the substrate into a beaker of methanol or ethanol.The substrate is held vertically and dipped into the solvent along thedirection of the sheet orientation. The substrate is then placed onto acloth and allowed to dry. This procedure of dipping and drying isperformed two more times. The CNT sheet has now been densified and has athickness of approximately 200 nm. Several layers of the hole-injectingpolymer PEDOT:PSS (the 8605 layers) are deposited after the solvent hasbeen dried from the densified CNT sheet. PEDOT:PSS is obtained from(Bayer Co.) and is filtered prior to depositing onto the CNT substrate.A first layer is deposited at a spin rate of 6100 rpm, with an initialacceleration of 21,500 rpm/second, for 20 seconds. A plastic syringe isfilled with the solution of PEDOT:PSS in water. The spinner is startedand the solution is immediately dropped onto the substrate whileacceleration is still occurring. This produces a thin film of PEDOT:PSS.The substrate is then baked at a temperature of 120° C. for 30 minutes.A second layer of PEDOT:PSS is deposited using the same spin process,but with a slower acceleration of 160 rpm/second (but the same targetrate of 6100 rpm and time of 20 seconds). The solution is dropped ontothe substrate prior to the start of spinning. This film is then bakedunder the previous conditions of 120° C. for 30 minutes. A third andfourth layer are deposited using the same spinning conditions as for thesecond layer (6100 rpm, 160 rpm/second acceleration, 20 seconds) andbaked after each deposition for 30 minutes at 120° C. The transport(8603) and emissive layers (8604) of the device are produced in a highvacuum chamber equipped with thermal deposition sources. The layers aredeposited by resistively heating tungsten or molybdenum boats containingorganic powders at a base pressure <2×10⁻⁶ Torr. The first layer (α-NPD,hole-transport layer: 8603) is deposited onto the existing CNT/PEDOT:PSSlayers at a constant rate of 1 Å/s until 700 Å are accumulated. Thesecond layer (Alq₃, emissive and electron transport layer: 8604) isdeposited at a rate of 1 Å/s until 500 Å are accumulated. The chamber isthen vented and a shadow mask used for cathode deposition is placed onthe substrate. The chamber is pumped again, and a bilayer cathode oflithium fluoride and aluminum is deposited onto the structure. The layerof lithium fluoride is deposited at a rate of 0.1 Å/s until a 10 Åthickness is accumulated. The final layer of aluminum is deposited at aninitial rate of 0.2 Å/s. Once 100 Å have been deposited, the rate isramped up to 5 Å/s over a 60 second period. This rate is held until 1200Å of aluminum have been deposited.

Example 68

This Example serves to illustrate a carbon nanotube-based PLED with abottom-up structure for construction on drive electronics foractive-matrix displays, as shown in FIGS. 87 and 88. The devicestructure is as follows: Aluminum/Calcium/MEH-PPV/PEDOT:PSS/CNT sheet.Fabrication of the above-described device starts with a substrate ofeither display glass: 8706 layer in FIG. 87 or n-type silicon (8806 inFIG. 88). The substrates are put onto a shadow mask and placed inside avacuum chamber equipped with tungsten or molybdenum sources forthermally evaporating metals. For the fabrication of the bottom-up OLEDof FIG. 87, a layer of aluminum (8705) is deposited at a rate of 5 Å/sto obtain a final thickness of 1000 Å. A 300 Å thick layer of calcium(8704) is then deposited, also at a rate of 5 Å/s. The metal-coatedsubstrate is removed from the vacuum chamber directly into an inertatmosphere so that the calcium film is not exposed to oxygen. Whileenclosed in the inert atmosphere, a film of MEH-PPV (8703) is spin-castonto the substrate from a solution of 0.2 wt % MEH-PPV in chloroform. Asyringe is used to drop the solution onto the accelerating spinner. Aspin speed of 3000 rpm with an acceleration of 3400 rpm/s and a durationof 30 seconds is used to deposit the film. The film is dried overnightbefore subsequent film deposition. After drying the film, several layersof PEDOT:PSS are added (8702). The first layer is deposited by startingthe spinner with an acceleration of about 160 rpm/s and immediatelydropping the solution onto the substrate from a syringe. The target spinrate for the spinner is 6100 rpm, and the duration of the deposition is20 seconds. Subsequent layers use the same spinning parameters, however,the solution is dropped onto the substrate before starting the spinner.After each film is deposited, it is baked for 30 minutes at 120° C. Forthe final step of making the device, the substrate withAI/Ca/MEH-PPV/PEDOT:PSS is place onto a stand, and a free-standing sheetof nanotubes (8701) is placed on top of the existing films. The sheet isnow weakly bound to the substrate. The entire substrate is heldvertically and dipped into a beaker of ethanol or methanol with thedipping motion in the same direction as the orientation of the nanotubesheet. This is repeated several times, allowing the solvent to dry inbetween dips.

Example 69

This Example serves to illustrate an essentially transparent PLED, basedon use of carbon nanotube sheets as both anode and cathode, as shown atFIG. 89. Nanotube anode is placed on display glass or flexible plastic,in accordance with some embodiments of the present invention. Such adevice comprises the following structure: CNT sheet/PEDOT:PSS/MEH-PPV/Calcium-coated CNT sheet. To fabricate such anabove-described device, free-standing carbon nanotube (CNT) sheet (8904)is placed on a substrate of Corning 1737 display glass (8905) orflexible plastic (e.g., PET or PEN). The sheet is weakly bound to thesubstrate and can be rubbed off very easily. Care must be taken duringsubsequent processing so that this does not occur. The substrate withthe CNT sheet on top is then densified. The densification process isperformed by placing the substrate into a beaker of methanol or ethanol.The substrate is held vertically and dipped into the solvent along thedirection of the sheet orientation. The substrate is then placed onto acloth and allowed to dry. This procedure of dipping and drying isperformed two more times. The CNT sheet has now been densified and has athickness of approximately 200 nm. Several layers of the hole-injectingpolymer PEDOT:PSS (8903) are deposited after the solvent has been driedfrom the densified CNT sheet. PEDOT:PSS (Bayer Co.) is filtered prior todepositing onto the CNT substrate. A first layer is deposited at a spinrate of 6100 rpm (with an initial acceleration of 21,500 rpm/second) for20 seconds. A plastic syringe is filled with the solution of PEDOT:PSSin water. The spinner is started and the solution is immediately droppedonto the substrate while acceleration is still occurring. This producesa thin film of PEDOT:PSS, but one which acts to provide enhancedadhesion of the CNT sheet to the substrate. The substrate is then bakedat a temperature of 120° C. for 30 minutes. A second layer of PEDOT:PSSis deposited using the spin process with a slower acceleration of 160rpm/second, but the same target spin rate of 6100 rpm and time of 20seconds. The solution is dropped onto the substrate prior to the startof spinning, however, unlike the first film for which spinning wasstarted before dropping the solution on. This film is then baked underthe previous conditions of 120° C. for 30 minutes. A third and fourthlayer are deposited using the same spinning conditions as used for thesecond layer (6100 rpm, 160 rpm/second acceleration, 20 seconds) andbaked after each deposition for 30 minutes at 120° C. Before adding theemissive layer (8902) it is necessary to coat a free-standing CNT sheetwith calcium. This is done in a vacuum chamber equipped with a tungstenor molybdenum source for evaporating metals. The chamber is pumped to abase pressure <2×10⁻⁶ Torr, and a layer of calcium is deposited at arate of 1 Å/s until a 300 Å thickness has been deposited. The emissivelayer (8902) of the device is produced by drop-casting a 0.15 wt %solution of MEH-PPV onto the substrate using a syringe. Instead ofspinning the film, the substrate is tipped vertically and the excesssolution is allowed to run off onto an absorbent, lint-free clean roomcloth. The substrate is positioned horizontally again and, while thefilm is still wet, the coated, free-standing CNT sheet (8901) is placedonto the film with the calcium side on the film. This is allowed to dryovernight in an inert atmosphere. After drying, the CNT nanofiber sheet(8901) is densified in inert atmosphere using a solution of methanol orethanol containing a very small concentration of MEH-PPV. The substrateis dipped vertically into the solution along the direction oforientation of the CNT sheet. The solvent is removed by drying forseveral hours, and the device is then ready for use.

Example 70

FIG. 90 depicts an organic solar cell or photodetector based on carbonnanotube ribbons as a front-surface transparent conducting electrode. Asshown in FIG. 90, an organic solar cell or photodetector can be formedon a glass substrate 9005. First, the aluminum electrode 9004 isdeposited on a glass substrate. ITO-coated glass substrates (<15 Ohm/sqwith ˜85% light transmission) were obtained from Delta Technologies Ltd.EL-grade PEDOT-PSS was purchased from Bayer AG. RR-P3HT and PCBM werepurchased from American Dye Source. All materials were used as receivedwithout further purification. Applicants fabricated four devices on eachsubstrate, each having an area of ˜9 mm². The ITO-coated glass substratewas etched and cleaned before being plasma-treated for five minutes (90seconds for flexible substrates) under O₂ gas. A layer of PEDOT:PSS 9003was then spin-coated onto the substrate at 6100 rpm creating a 30-35 nmlayer. The sample was then dried by being heated at ˜120° C. for 100minutes (60 minutes at 110° C. for flexible substrates) in a glove box.The photoactive material solution was dispersed by a magnetic stirrerfor 3-7 days until it was optimally dispersed. The solution was thenspin-coated onto the sample at 700 rpm creating a 50-60 nm layer using atoluene solution consisting of roughly a 1:2 ratio of PCBM and RR-P3HT.The final layer was made up of 65% RR-P3HT and 35% PCBM 9002. A carbonnanotube ribbon 9001 was then deposited. A surface profiler (AMBIOSXP-1) was used to measure film thickness. The finalized device was thenannealed on a hot plate in a glove box at the desired temperature for adesired amount of time. The organic semiconductor or mixture of organicp-type and n-type semiconductors were deposited on this electrode. Thesolar cell was completed by application of the carbon nanotube nanofibersheet. Nanotube sheets can be optionally densified using surface tensioneffects of an imbibed liquid. The rapid evaporation of the solventabsorbed in the sheet causes shrinkage in the sheet thickness directionleading to densification.

Example 71

As shown in FIG. 90, an organic solar cell or photodetector is formed ona plastic substrate (polyethylene naphthalate or polyethyleneteraphthalate) as in Example 70 and in accordance with embodiments ofthe present invention.

Example 72

FIG. 91 shows a solar cell or photodetector based on carbon nanotuberibbons as a bottom transparent conducting electrode on a glasssubstrate. First, the aligned carbon nanotube sheet 9104 is deposited ona glass substrate 9105. An SEM image of enlarged aligned carbon nanotubesheet 9104 is shown on the left side of FIG. 91. Nanotube sheets can beoptionally densified using the surface tension effects of an imbibedliquid. The rapid evaporation of the solvent absorbed in the sheetcauses shrinkage leading to densification. A layer of PEDOT:PSS 9103 wasthen spin-coated onto the substrate at 6100 rpm creating a 30-35 nmlayer. The sample was then dried by being heated at ˜120° C. for 100minutes (60 minutes at 110° C. for flexible substrates) in a glove box.The organic semiconductor or mixture of organic p-type and n-typesemiconductors 9102 are then deposited on this electrode as in Example70. The solar cell is completed by application of the aluminum electrode9101 (typically by thermal vacuum deposition).

Example 73

This Example serves to illustrate a solar cell or photodetector based oncarbon nanotube ribbons as a bottom transparent conducting electrode ona flexible plastic substrate (polyethylene naphthalate or polyethyleneteraphthalate) as in Example 72. First, the carbon nanotube sheet isdeposited on a glass substrate. Nanotube sheets can be optionallydensified using the surface tension effects of an imbibed liquid (seeabove). The rapid evaporation of the solvent absorbed in the sheetcauses shrinkage leading to densification. The organic semiconductor ormixture of organic p-type and n-type semiconductors are deposited onthis electrode as in Example 70. The solar cell is completed byapplication of the aluminum electrode (typically by thermal vacuumdeposition).

Example 74

This Example serves to illustrate the fabrication of a transparent solarcell with two transparent CNT sheets, in accordance with someembodiments of the present invention. FIG. 92 demonstrates a transparentsolar cell or photodetector based on carbon nanotube ribbons/sheets astop and bottom transparent conducting electrodes. First, the carbonnanotube sheet 9204 is deposited on a solid glass or flexible plasticsubstrate 9205. Nanotube sheets can be optionally densified using thesurface tension effects of an imbibed liquid. The rapid evaporation ofthe solvent absorbed in the sheet causes shrinkage leading todensification. A very thin Al layer 9203 (<100 nm) is then deposited ontop of the CNT sheet. An organic semiconductor or mixture of organicp-type and n-type semiconductors 9202 are deposited on this Al-coatedelectrode. Owing to the small organic layer thickness, the activeorganic layer is also transparent. The transparent device is completedby application of the carbon nanotube sheet 9201. Nanotube sheets can beoptionally densified using the surface tension effects of an imbibedliquid. The rapid evaporation of the solvent absorbed in the sheetcauses shrinkage leading to densification. The transparent solar cells,for example, can be used in smart windows, displays, or smartmultifunctional ceilings and covers for buildings.

Example 75

FIG. 93 demonstrates a tandem solar cell or photodetector based oncarbon nanotube ribbons/sheets as a top transparent conductingelectrode. First, the bottom reflective aluminum electrode 9302 isdeposited on a substrate 9301. The organic semiconductor or mixture oforganic p-type and n-type semiconductors 9303 are deposited on thiselectrode. The first solar cell is completed by application of a carbonnanotube sheet 9304 as a transparent separation layer (also calledcharge recombination layer). Nanotube sheets can be optionally densifiedusing the surface tension effects of an imbibed liquid. The rapidevaporation of the solvent absorbed in the sheet causes shrinkage in thethickness direction, leading to densification. A thin aluminum layer9305 can be optionally deposited on a nanotube sheet electrode beforeapplication onto first cell to reduce voltage losses. The second layerof organic semiconductor or mixture of organic p-type and n-typesemiconductors 9306 are deposited on this electrode. The tandem solarcell is completed by application of a top carbon nanotube sheet 9307.Again, such nanotube sheets can be optionally densified using thesurface tension effects of an imbibed liquid.

Example 76

This example demonstrates the additional functionality of carbonnanotube charge collectors for enhancement of light absorption andcharge generation due to three-dimensionality and a nano-antenna effectin the solar cells (see FIG. 94). The large interface and nanoscalemorphology between organic material and carbon nanotubes significantlyimproves the photo induced charge transfer, charge separation andcollection. There is experimental evidence (FIG. 94) that spectralsensitivity is broadened due to additional photo induced charge transferbetween organic material and carbon nanotubes. The extended and alignednature of CNT sheets can improve the morphology between the acceptor anddonor materials and extend the spectral sensitivity into the infraredrange. It is known that localized plasmon excitations can occur in smallmetal nanoparticles by direct light absorption due to simpler selectionrules. The excited plasmon can lead to an increased amount ofphotoexcited electrons in the metal which are capable of surmounting theSchottky barrier. For this purpose, carbon nanotubes decorated withmetal nanoparticles have been examined. The high electric field strengthin the vicinity of the excited surface plasmon can also lead toincreased absorption of photons in the organic matrix. Thus, the effectof surface plasmons can lead to enhancement of photocurrent in organicsolar cells in spectral regions where an organic material does notabsorb much. The plasmon excitations in carbon nanotubes and metalcoated nanofibers can enhance the UV spectral sensitivity of CNT-basedsolar cells and photodetectors. The absorption spectra were measured ona Perkin-Elmer Lambda 900 UV-Vis-NIR Spectrophotometer. Thecurrent-voltage characteristics were recorded with a Keithley 236source-measure unit. A solar simulator (150 W Xenon lamp with AM0 andAM1.5 filters from Spectra-Physics and focusing lens), with lightintensity calibrated at 100 mW/cm², was used as the light source forsolar cell efficiency measurements. The reported efficiency measurementwas not corrected for spectral mismatch.

Example 77

This example shows a transparent carbon nanotube sheet embedded inpolarization-sensitive photodetectors. The high anisotropy of thealigned CNT sheet allows increasing the polarization ratio by 25%, asshown at I-V curve in FIG. 97. A broadband (300 nm-10 μm) polarizationsensitive photodetector comprises carbon nanotube ribbons/sheets as atop transparent polarizing conducting electrode and an aluminum bottomelectrode deposited on a glass or plastic substrate. The organicsemiconductor or mixture of organic p-type and n-type semiconductors aredeposited on this electrode. The high polarization ratio is achieved bymechanical rubbing of the organic layer or by other means (e.g.,electric field alignment). The solar cell is completed by application ofthe carbon nanotube sheet. Nanotube sheets can be optionally densifiedusing the surface tension effects of an imbibed liquid. The rapidevaporation of the solvent absorbed in the sheet causes shrinkage in thethickness direction leading to densification. The polarization ratio canbe additionally increased by placing an additional CNT sheet on the Albottom electrode to polarize the light reflected back from the Alelectrode.

Example 78

This example shows a method of improving electrical conductivity andmodulating the work function of carbon nanotube sheets by deposition ofthin films of high work function metals such as Au or Pt on carbonsheet, as shown in FIG. 98. Sheet resistance can be lowered at least 5times to 100 ohm/square. Coated by high work function metals such as Auor Pt, free-standing CNT sheets can be used as anodes for solar cells.FIG. 98 shows SEM image of Au/Pd-coated carbon nanotubes sheet.

Example 79

This example illustrates a method of improving electrical conductivityand modification of work function of carbon nanotube sheets bydepositing thin films of low work function metals such as Al or Ca onsuch carbon sheets. Sheet resistance was lowered at least 5 times to 100ohm/square. Coated by low work function metals such as Al or Ca,free-standing CNTs can be used as cathodes for solar cell.

Example 80

In some embodiments, the present invention can be broadly applied to athin transparent organic transistor or phototransistor which comprisestransparent CNT sheet electrodes, thin transparent under-gate insulator(inorganic or organic), and an organic photoactive layer, and can beused in the fabrication of a variety of transparent integrated circuits,including those for display applications and photodetectors. FIG. 99depicts a transparent phototransistor based on carbon nanotuberibbons/sheets as a bottom transparent conducting electrode. First, thebottom transparent CNT sheet electrode 9902 is deposited on a substrate9901. A transparent gate dielectric 9903, such as SiO₂ or Al₂O₃ orpolymeric insulator such as parylene, is coated on top of electrode9902. The source 9904 and drain electrodes 9905 are formed by placingtwo transparent carbon nanotube ribbons on top of the insulating layer.The transparent phototransistor is completed by deposition of organicsemiconductor or mixture of organic p-type and n-type semiconductors9906 on top of the structure. Nanotube sheets can be optionallydensified using the surface tension effects of an imbibed liquid. Theevaporation of the solvent absorbed in the sheet causes shrinkage in thethickness direction, leading to densification. The transparency of thisdevice enables photomodulation of source-drain current, which is usefulfor optical chip-to-chip information transfer.

Example 81

This Example serves to illustrate the preparation of a transparentnanofiber-metal oxide photoelectrode, in accordance with someembodiments of the present invention. Transparent nanotube sheets madeby the process of Examples 18 and 21 were deposited as an aligned sheetstack on a glass substrate. Because the nanotubes are well aligned inthe sheet, the electrical and optical properties are anisotropic.Referring to FIG. 101, the transparent porous nanofiber electrode 10102was fabricated according to the present invention. A semiconductorphotoelectrode 10103 containing the metal oxide nanoparticles (forexample, 10-20 nm TiO₂ nanoparticles) was coated on the rough surface ofthe transparent porous nanofiber electrode 10102 by a printing method orsol-gel method to form a film of approximately 10-20 μm thickness.Sintering and anatase phase formation was performed at 450° C. in theinert atmosphere for 30-60 minutes. After sintering, the layer exhibitsa porosity of 0.5-0.65. A monolayer of red dye molecules wasincorporated in the highly porous nanofiber-metal oxide electrode byimpregnation, for example, with an absolute ethanol solution of theruthenium dye-IIcis-dithiocyanato-N,N′-bis(2,2′-bipyridyl-4,4′-dicarboxylicacid)-(H₂)(TBA)₂RuL₂(NCS)₂ (H₂O)₄ sensitizer at a concentration of 20 mgof dye per 100 ml of solution. The impregnation process can be done atroom temperature overnight. The electrode was rinsed with ethanol andthen dried.

Example 82

This Example serves to illustrate the preparation of a transparentnanofiber-metal oxide photoelectrode by microwave irradiation, inaccordance with some embodiments of the present invention. Referring toFIG. 101, the transparent porous nanofiber electrode 10102 on plasticsubstrate was fabricated according to the Example 81. A semiconductorphotoelectrode 10103 containing the metal oxide nanoparticles (forexample, 10-20 nm TiO₂ nanoparticles) was coated on the rough surface ofthe transparent porous nanofiber electrode 10102 by a printing method orsol-gel method to form a film of approximately 10-20 μm thickness. Rapidsintering and anatase phase formation was performed by a multi-modemicrowave heating at a frequency chosen from the range varying from 2 to30 GHz and power of about 1 kW for 5 minutes. After sintering, the layerexhibits a porosity of 0.5-0.65. A monolayer of red dye molecules isattached to the surface of the highly porous nanofiber-metal oxideelectrode by impregnation, for example, with an absolute ethanolsolution of the Ruthenium dye-IIcis-Dithiocyanato-N,N′-bis(2,2′-bipyridyl-4,4′-dicarboxylicacid)-(H₂)(TBA)₂RuL₂(NCS)₂(H₂O)₄ sensitizer at a concentration of 20 mgof dye per 100 ml of solution. The impregnation process can be done atroom temperature overnight. The electrode was rinsed with ethanol andthen dried.

Example 83

This Example serves to illustrate the preparation of a nanofiberreduction electrode, in accordance with some embodiments of the presentinvention. The transparent porous nanofiber reduction electrode 10105 ofFIG. 101 was fabricated according to embodiments of the presentinvention. No platinization of counter-electrode is necessary becausethe nanofibers work both as a charge collecting electrode and as acatalyst.

Example 84

This Example serves to illustrate the preparation of a dye-sensitizedsolar cell, in accordance with some embodiments of the presentinvention. The dye-sensitized solar cell is prepared by usingsemiconductor photo electrode and the nanofiber sheet reductionelectrode obtained from Examples 81-82. These two electrodes aremaintained at an inter-electrode separation of 50-100 μm by using aspacer. The charge conducting agent is introduced in the space betweenthese two electrodes and sealed. The charge conducting agent can be aredox couple in a liquid (including the following redox electrolytes: 1)iodolyte TG-50 (from Solaronix); 2) one based on methoxypropionitrilewhich has a low viscosity but low boiling point (bp 163-165 C); 3) onebased on gamma-butyrolactone which also has low viscosity but higherboiling point (bp 206 C)) or quasi solid-state electrolyte, orsolid-state hole conductor such as conjugated polymers (such aspoly(3-hexylthiophene) or small organic molecules (such as TPD). Theelectrons generated from the photosensitive dye or quantum dots can beeasily be transferred to an external circuit by using a highlyconducting nanoporous nanofiber sheet comprising three-dimensionallydistributed nanofibers. One optionally preferred is the carbon nanofibersheets of invention embodiments. In addition, the electrodes can be madeflexible and lightweight by using thin flexible transparent substrates.Such solar cell can be utilized in portable devices.

Example 85

This Example serves to illustrate the preparation of a transparentdye-sensitized solar cell (DSC), in accordance with some embodiments ofthe present invention. The dye-sensitized solar cell was prepared byusing transparent semiconductor photoelectrode and the transparentreduction electrode obtained from Examples 81-83. To have a transparentDSC, the thickness of the top photoelectrode was limited by 10 μm. Thethickness of the multi-walled carbon nanotube nanofiber ribbons/sheetsis limited by 1-2 layers. Both electrodes maintain a spacing of 10-100μm by a spacer. A charge conducting agent was introduced into the spacebetween two electrodes and sealed. The charge conducting agent can be aredox couple in a liquid (including the following redox electrolytes: 1)iodolyte TG-50 (from Solaronix); 2) one based on methoxypropionitrilewhich has a low viscosity but low boiling point (bp 163-165° C.); 3) onebased on gamma-butyrolactone which also has low viscosity but higherboiling point (bp 206° C.) or a quasi solid-state electrolyte, or asolid-state hole conductor, such as conjugated polymers (such aspoly(3-hexylthiophene) or small organic molecules (such as TPD). Theelectrons generated from the photosensitive dye or quantum dots can bemore easily transferred to an external circuit by a 3-dimensionaldistributed nanofiber electrode. In addition, the electrodes can be madeflexible and lightweight by using thin flexible transparent substrates.Such solar cells can be utilized in portable devices.

Example 86

This example demonstrates that the performance of a dye-sensitized solarcell (DSC) can be improved by using the catalytic electrochemicalproperties of single wall nanotubes by coating them on the transparentanode made of multiwall nanotube transparent sheets of the presentinvention (see SEM image of such electrode in FIG. 102). By thisprocess, Applicants show the enhancement of the electrochemical chargetransfer of holes to the SWNT-coated anode. This example also describesa method for fabricating a composite nanotube sheet that is an effectiveanode for a photoelectrochemical device, i.e., a Gräetzel cell. Aruthenium dye-II cis-dithiocyanato-N,N-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)-(H₂)(TBA)₂RuL₂(NCS)₂(H₂O)₄(Dyesol, B2/N719) was used as a sensitizer for the TiO₂ particles. Theprepared solution was 3×10⁻⁴ M Ru (-II) dye in absolute ethanol(CH₃CH₂OH). The chemical structure of the dye is RuC₅₈H₈₆N₈O₈S₂.4H₂O.The electrolyte was iodide-based redox electrolyte-iodolyte TG-50 (from“Solaronix”). Porous titania, TiO₂ (from “Dyesol”) coated conductingsubstrate, fluorine-doped SnO₂ over layer with transmission >85% wasused as the photo-electrode in assembling DSC cells. Before sensitizingthe TiO₂, it was sintered at 450° C. for 30 minutes. The annealed filmwas immersed in a 3×10⁴ M Ru (-II) dye containing ethanol solutionovernight (˜12 hours) at room temperature. The TiO₂ electrode was dippedinto the dye solution while it was still hot, i.e., its temperature wasabout 80° C. After the completion the dye adsorption, the electrode wasdried under a stream of nitrogen, N₂. The typical electrolyte wasiodolyte TG-50 (from Solaronix), but other electrolytes also have beentested. The counter electrode was prepared of aligned multiwall carbonnanotube (MWNT) sheets on a glass substrate. On top of the sheet, asuspension of single wall nanotubes (SWNTs) was deposited by a 3 stepmethod: (i) vacuum-filtering a dilute, surfactant-based suspension ofpurified nanotubes onto a filtration membrane (forming a homogeneousfilm on the membrane); (ii) washing away the surfactant with purifiedwater; and (iii) dissolving the filtration membrane in solvent. Thedissolvable filter was from Millipore (GS-0.22 micron). About 0.3 mg ofSWNTs (HiPco from Carbon Nanotechnologies Inc.) in a 100 mL surfactant(Triton-1000) solution was used to make the SWNT dispersion. 3 mL ofSWNT dispersion was added to 500 mL deionized water to make nanotubeink. Having prepared the necessary materials, photo and counterelectrodes were placed together offsetting them against each other. Thestrip of each electrode served as contact points. In order to hold theelectrodes together, two binder clips were used.

Example 87

This example shows that the performance of DSC can be further increasedby using multiple nanotube sheets: Measurements show that currentdensity—voltage characteristics of the MWNT sheet are comparable to thecurrent density—voltage characteristic obtained when using only ITO as acounter electrode. The sheet of MWNTs works as an electrochemicalcatalyst, but due to high serial resistance of very thin electrodes theefficiency is lower than that for conventional ITO-based DSC. If,however, the number of MWNT sheets is increased, measurement showshigher electrochemical activity in the redox electrolyte system and theshort circuit current (I_(sc)) increases with increasing number of MWNTsheets. Because of the increase in the number of layers, the contactarea of MWNTs with electrolyte increases. Additionally, the serialresistance of the electrode decreases.

Example 88

This Example serves to illustrate the preparation of a tandem solarcell, in accordance with some embodiments of the present invention. FIG.103 illustrates a multijunction solar cell (or tandem solar cell) inwhich the top transparent electrode is sketched as 10301, together withtransparent separation layer 10304 between the layers of single junctionsolar cells having selective spectral sensitivity to visible (cell 1,parts 10301-10304) and near infrared parts of solar spectrum (cell 2,parts 10304-10308). The tandem solar cell comprises two compartmentcells. The top compartment is a dye- or quantum dot sensitized solarcell prepared by method described in Example 83. The porous metal oxidelayer 10302 in the first compartment comprises nanoparticles with anaverage diameter of 10-20 nm. A monolayer of the red dye molecules 10308(Ruthenium dye-IIcis-Dithiocyanato-N,N′-bis(2,2′-bipyridyl-4,4′-dicarboxylicacid)-(H₂)(TBA)₂RuL₂(NCS)₂(H₂O)₄) is attached to the surface of thehighly porous nanofiber-metal oxide electrode by impregnation, forexample, with an ethanol solution of the RuC₅₈H₈₆N₈O₈S₂.4H₂O. Thisporous layer 10302 was then impregnated with redox electrolyte. The twocompartments are separated with a transparent electrically-conductingnanofiber sheet 10304 imbedded in a transparent non-porous metal oxidematrix. This transparent separation electrode has a transparency of atleast 70% in the region of interest. The second compartment is aninfrared quantum dot sensitized solar cell prepared by method describedin Example 83. The porous metal oxide layer 10305 in the secondcompartment comprises nanoparticles with an average diameter of 50-300nm. A monolayer of the infrared quantum dot sensitizer is attached tothe surface of the highly porous nanofiber-metal oxide electrode byimpregnation with a hexane solution or by growing the quantum dots insitu inside pores of the metal oxide matrix. The infrared quantum dotsensitizers include, for example, PbSe or PbS semiconductor nanocrystalspossessing charge multiplication properties. This porous layer was thenimpregnated with redox electrolyte 10306.

Example 89

This Example serves to illustrate the preparation of a tandem solar cellwith a double sheet charge separation layer, in accordance with someembodiments of the present invention. The dye-sensitized tandem solarcell was prepared by using a semiconductor photo electrode and thereduction electrode obtained from Example 88. A double sheet chargeseparation layer in a tandem, with a transparent nanotube sheet ofp-type (high work function) facing the first junction, while the n-type(low work function) nanotube sheet coated on first one acts as anelectron collecting layer from the second junction. Electricalconductivity and modification of the work function of the chargeseparation layer in a tandem is accomplished by deposition of thin filmof gold Au metal (50 nm) on the carbon sheet on one side (which decreasesheet resistance 5 times to 100 ohm/square) and the other side is coatedby a low work function metal such as Al or Ca.

Example 90

This example shows that an elastomerically deformable nanotube sheet canbe made by the process of Example 32 and then overcoated with a secondelastomeric silicone rubber sheet, while retaining the ability of thenanotube sheet to be elastomerically deformed without producing asignificant dependence of nanotube sheet resistance on elongation of thenanotube sheet. The importance of this demonstration is that it enablesthe fabrication of high strain actuator stacks comprising more that onelayer of actuator material (such as an electrostrictive rubber likesilicone rubber) and more than two electrodes. After fabrication of thesilicone-rubber-attached elastomeric nanotube sheet of Example 32,stretch was relaxed and a second silicone rubber sheet was attached ontop of the carbon nanotube sheet by liquid coating with a siliconerubber resin followed by setting of the silicone liquid resin. Afterthis process (which produced an elastomerically deformable nanotubesheet between two silicone rubber sheets), the inventors found that thenanotube sheet (and associated silicone rubber sheets) can be highlyelongated without causing a significant change in the resistance of thenanotube sheet. This process can be conveniently extended to produceelastomerically-deformable stacks comprising one or more nanotubeelectrode sheets that are laminated between sheets of elastomer, whereinthe number of alternating nanotube sheet electrodes and elastomer sheetelectrodes is arbitrarily large. The method of this example and Example32 can be used to produce inflatable balloons containing one or morelayers of conducting nanotube sheets. To start the process of conductingballoon formation, the initial inner balloon layer can optionally beinflated using a gas or liquid or formed on a mandrel will innon-inflated state, and subsequently un-expanded before application ofthe first nanotube sheet.

Example 91

This example describes a flexible and or elastomeric transparentantenna, shown schematically at FIG. 63. Such antenna can be made bylaminating the optionally rigid, or flexible (and/or elastomeric)insulating substrate with the following components: a remote feeder, ina form of a microstrip line, such a feeder been separated by a thininsulating layer and located below the radiator/receiver layer, which ismade of transparent oriented nanofiber ribbons that are laminated on aflexible or elastomeric. Such antenna, in a receiving mode, can be usedin radio-frequency identification (RFID) systems. Reflecting thetransparency of nanofiber sheets of invention embodiments, thenanofiber-sheet-based antennas can be transparent. This transparency canenable, for instance, readability of underlying bar code linesassociated with the tag. The device configuration and components areshown in FIG. 63. Component 6301 of said antenna is composed ofsubstrate 6301, aligned nanotube sheet radiator/receiver plate 6302, andnanotube sheet or yarn array feeder 6303, and the thin insulatingseparation layer 6304. The patterning of sheet 6302 will allow one toadjust the frequency of the antenna. Such optionally elastomericantennas can be made of one uniform size, and then stretched to obtain adesired size and desired antennae properties.

All patents and publications referenced herein are hereby incorporatedby reference. The invention being thus described, it will be obviousthat the same may be varied in many ways. Such variations are not to beregarded as a departure from the spirit and scope of the invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1-469. (canceled)
 470. A process of producing ribbons or sheetscomprising nanofibers, the process comprising: (a) providing apre-primary assembly comprising a substantially parallel array ofnanofibers oriented in a direction; and (b) drawing the nanofibers fromthe array of nanofibers to form a ribbon or sheet having an alignmentdirection, wherein the alignment direction of the ribbon or sheet is atan angle to the direction of the array of nanofibers at no more than90°.
 471. The process of claim 470, wherein the nanofibers comprisecarbon nanotubes.
 472. The process of claim 470, wherein the nanofiberscomprise carbon nanotubes that have an interior that are, at leastpartially, filled with a material.
 473. The process of claim 470,further comprising infiltrating the ribbon or sheet with a liquid andsubsequently evaporating the liquid from the ribbon or sheet wherein theinfiltration and evaporation at least partially densifies the ribbon orsheet and forms a densified carbon nanotube ribbon or sheet.
 474. Theprocess of claim 470, wherein said infiltration with the liquidcomprises a method selected from the group consisting of vaporcondensation, imbibing a liquid, exposure to an aerosol of a liquid, andcombinations thereof.
 475. The process of claim 473, wherein the liquidcomprises a substance selected from the group consisting of acetone,ethanol, methanol, isopropyl alcohol, toluene, chloroform,chlorobenzene, liquids having similar cohesive energy densities, andcombinations thereof.
 476. The process of claim 470, wherein at leastthe drawing is conducted at an elevated temperature using a heatingmeans selected from the group consisting of (a) resistive heating of thenanofibers by conducting current along the ribbons or sheets; (b) theabsorption by the nanofibers of electromagnetic radiation from a regionselected from the group consisting of visible, ultraviolet, infrared,radio frequency, microwave frequency, and combinations thereof; and (c)combinations thereof.
 477. The process of claim 470, wherein thepre-primary assembly is formed by growing a nanofiber forest on asubstrate using a catalyst.
 478. The process of claim 477, wherein thedirection of the nanofibers in pre-primary assembly is approximatelyorthogonal to the substrate.
 479. The process of claim 477, wherein thesubstrate has a first side and a second side, and wherein forests ofnanofibers are grown on both the first side and the second side. 480.The process of claim 470, wherein the nanofiber array comprises a forestof nanofibers, and wherein the forest of nanofibers is stripped from agrowth substrate for the forest and produced into the ribbon or sheetwhile not attached to said growth substrate.
 481. The process of claim480, wherein more than one forest of nanofibers is stripped from growthsubstrates for the forests of nanofibers and stacked upon each other toprovide an array of nanofiber forest layers from which the ribbon orsheet is produced.
 482. The process of claim 477, further comprisingconnecting an attachment to a sidewall of the nanofiber forest or nearthe sidewall.
 483. The process of claim 482, wherein the attachment isselected from the group consisting of adhesives, arrays of pins, or acombination thereof.
 484. The process of claim 470, wherein thenanofibers are drawn without substantially twisting the ribbon or sheet.485. The process of claim 470, further comprising applying an enhancingagent to the ribbon or sheet, wherein the enhancing agent comprises apolymer selected from the group consisting of polyvinyl alcohol, apolyvinyl alcohol copolymer, and combinations thereof.
 486. The processof claim 485, wherein the enhancing agent is applied to the ribbon orsheet while the ribbon or sheet is subjected to tensile strain.
 487. Theprocess of claim 470, wherein electrical conductivity of the ribbon orsheet is increased by incorporating an electrically conducting materialwith the ribbon or sheet, and wherein the electrically conductingmaterial is selected from the group consisting of conducting polymers,metals, metal alloys, and combinations thereof.
 488. The process ofclaim 470, further comprising coating the pre-primary assembly with ahydrophobic material.
 489. The process of claim 488, wherein thehydrophobic material comprises a fluorocarbon polymer.
 490. The processof claim 470, further comprising infiltrating the ribbon or sheet with acatalyst and growing the nanofibers within the sheet or ribbon bychemical vapor deposition.
 491. The process of claim 470, wherein thenanofibers are synthesized as a forest in a furnace growth region on asubstrate the continuously moves from a furnace growth region into aregion where the nanofibers in the forest are drawn.
 492. The process ofclaim 491, wherein the substrate is selected from the group consistingof a belt, a substrate attached to a belt, a drum, a substrate attachedto a drum, and combinations thereof.
 493. The process of claim 470,further comprising subjecting the ribbon or sheet to radiation selectedfrom the group consisting of electron beam, ion beam, microwave, radiofrequency and combinations thereof.
 494. The process of claim 470,wherein the drawing of the nanofibers forms a ribbon and the ribbon islaminated to form a sheet.
 495. The method of claim 494, wherein thelaminating involves the use of at least one of a chemical binding agent,an organic polymer, an electron beam, thermal treatment, an ion beam,radio frequency, irradiation, microwave irradiation, and combinationsthereof.
 496. A nanofiber ribbon or sheet made by the process of (a)providing a pre-primary assembly comprising a substantially parallelarray of nanofibers oriented in a direction, and (b) drawing thenanofibers from the array of nanofibers to form a ribbon or sheet havingan alignment direction, wherein the alignment direction of the ribbon orsheet is at an angle to the direction of the array of nanofibers at nomore than 90°, wherein the nanofiber ribbon or sheet is attached to asubstrate.
 497. The nanofiber ribbon or sheet of claim 496, wherein saidsubstrate comprises a material selected from the group consisting ofplastics, adhesive coated tapes, glass, metal, paper, and combinationsthereof.
 498. Plied layers comprising nanofiber ribbons or sheets madeby the process of (a) providing a pre-primary assembly comprising asubstantially parallel array of nanofibers oriented in a direction, and(b) drawing the nanofibers from the array of nanofibers to form a ribbonor sheet having an alignment direction, wherein the alignment directionof the ribbon or sheet is at an angle to the direction of the array ofnanofibers at no more than 90°, wherein the nanofiber ribbons and sheetsare each oriented but do not have the same preferential direction ofalignment of the carbon nanotubes.